Will leaking valve seals harm an engine?

If you mean the seals on the valve stems, basically the only factor is burning oil. I had a 1994 GMC Sonoma with the Vortec V 6 engine. Cheap rubber "O" rings on the valve stems failed at 50 K miles . I sold the truck when it had about 100 K miles and I know the subsequent owner drove many more miles without replacing the valve seals. Oil consumption was not much , but there was a big puff of blue smoke on each start caused by the oil that ran down the stems when the engine was off.