Blocking knitting using the washing machine?

Blocking is usually a one-time process for knitwear. It's used to even out the stitches in a single piece and to stretch several pieces to make the seams fit exactly before sewing them together. If your knitting is tight and even enough for your taste or the pieces fit just fine, you can get away without blocking at all.

Blocking is especially useful for delicate and "open" knitting patterns with holes, like lace, net or ajour / open work. The less tight your knitting is, the more it will benefit from blocking. The steaming (or heating with an iron) of a just finished, blocked piece of knitting sets the fibres of the yarn into their current shape, "locking" the loops and curves of the knitting into the yarn.

In my experience, blocking only needs to be repeated for lace doylies and similar pieces with a delicate geometric pattern. A sweater won't change it's shape much after the initial blocking (given it doesn't stretch due to vertical hanging). The best advice I'd give the person receiving the knitting it to first load the drying rack with whatever laundry there was in the machine and then lay the knitwear flat on top. If the knitwear has the tendency to shrink a little (I'm not talking about the shrinkage of wool, but some cotton sweaters have very tight sleeves when washed that stretch to their usual size after some wear), the person can gently stretch the wet sweater on the drying rack. That simulates blocking without all the efford involved.