Does dimming preserve the life of LEDs?

Here's what doesn't apply to you, but does apply to most consumer LEDs (sold as primary lighting devices, i.e. screw in bulbs, fluorescent replacements etc):

They are significantly overdriven from spec, for instance an LED will be sold as a 10 watt emitter, however the datasheet will spec it for baseline performance at 1050ma at around 3.3 volts - now hold on, that's only 3.6 watts, not 10! As you read on, you discover they achieve the "10 watt" nameplate by driving it at 2800ma at 3.6 volts, which is allowed, but with lower efficiency and higher junction temperatures requiring bigger heatsinks. Of course that overdrive figures into the LED's life, as the higher junction temperature is one factor in lamp aging.

The other factor is the driver circuit which converts 120/230VAC into the low-voltage, constant-current the LED emitter requires. For a packaged consumer product, this driver circuit is the weak link in the chain, and is the likely cause of death, especially since there are so many ways for bottom shelf brands (FE, UT, LoA) to cut corners here.

In that light, here's what does apply to your LED strip accent lighting.

Those strips usually run on 12VDC or sometimes 24VDC, but they don't include that complex driver circuit. They use a simple, bulletproof resistor for current limiting. Those strips don't have cooling fins, so they don't overdrive the LEDs much. The resistor is also sized for worst-case 14V (or 28V), which is what a car's alternator is putting out to charge the battery. At actual 12V they derate considerably - I've seen a 16 foot "24 watt" strip drop to 15 watts actual. These factors combine to make them dimmer than they could be, but conversely, more long-lasting.

Generally, DC LED strip installations have an AC power supply that is a separate item. It is a failure point, but you can replace it separately.

The other thing is, people really have no idea how long LEDs will actually last. The numbers they throw down are guesses. Realistically they will probably last a lot longer than claims made, but the claims are tempered to avoid appearing unrealistic. My rule of thumb is your installation won't live long enough to see the LED fail - you'll modernize, sell the home or die!

The upshot is, you're already in pretty good shape, and so dimming will help, but not a lot.

I would say the bigger reason to dim it is that lights bright enough to be useful workspace lighting would be annoyingly bright as ambience lights.

The answer depends more on the (triac) dimmer. LEDs have a sweet spot for voltage and will operate for a long time in the sweet spot. LED/triac dimmers actually just turn on and off rapidly. LEDs can handle the on/off better than other kinds of light, but the "flickering" would be the factor in break-down. At 0V obviously the LED would last for a very long time so when the LED is off for an extended amount of time (dimmer on low- like 10%) the light should last longer. But at half dim/brightness the dimmer is switching between 0V and (say) 3V and the damage due to flickering is probably somewhat significant. Theoretically, the LED would last twice as long, because it is off half the time, but turning off and turning on means the voltage will be increasing and decreasing; even though it is a rapid transition, time will be spent driving the LED at below specification (say 0.1 - 1.9 volts). This may or may not cause the LED to fail prematurely; remember, at 0V the LED will last even longer than it would at 3V.

The only way to know would be to test the lights at various "dimmings"... with different dimmers. The more rapidly the circuit/dimmer can go from 0V to 3V to 0V, the less time the LED will be diven at below specifification. A good quality LED dimmer might allow the LED to last longer, a cheap LED dimmer might cause premature failure... depending on the breadth of the LED's voltage specifications.

Before talking about LED lets talk incandescent lamps. If one includes a rectifier inline with the lamp it would driven by positive/ negative (depending on rectifier direction) pulses of a sinusoidal waveform. The negative pulses (our example) are blocked by the rectifier. Thus the lamp is off half the time. At 60 Hz that would be off every 120th of a second. The lamp appears to be about 25 to 30% less rather than 50%. That is because the filament of the lamp dims down but not off during the off cycle. Thus when the current returns it takes less because the filament is partially still hot and requiring less inrush current. This configuration can allow the incandescent lamp to have a life expectancy (LE) of 2 to four times longer than normal (my experience). Factors that would affect LE would be the grid, quality of the lamp and the robustness of the filament suspension (number of supports). It is better for the filament to wiggle less (rough service). Now as to the Led LE and dimming. The lamp does not dim completely every 120th of a second similar to the Incandescent lamp.This could be due to the nature of the led array or perhaps some capacitance in the driver circuit. Either way the lamp would have less inrush current than starting cold from 0 volts and be on half the normal time. Again the quality and power source will be critical to its LE. I currently have no direct LED LE experience but would expect the LE to greater due to LED observation and Incandescent lamp experience.