In Terraria I've gathered a few Life Crystals and haven't taken the time to use them. Is there ever a time I shouldn't use a Life Crystal right away?
Besides the reason BBz points out (you're already at max life) the only reason I can think of not to use a heart is if you already have 9 and you don't feel prepared to face the first real "boss" monster - the Eye of Cthulhu. Among other triggers, the Eye only spawns after you've got 10 hearts (200 life) total.
Dying to this boss isn't a big deal, and dying once to it causes it to leave. However, once it starts spawning it has a chance (1 in 3) of arriving each night until you've defeated it.
If used at maximum [health], a Life Crystal will not have any effect, though the same animation will commence. Doing this won't consume the Life Crystal, nor will it restore any health.
If you're in need of money, you can sell them for 1 gold and 50 silver.
Otherwise, save them for your friends who are starting out!
As of update 188.8.131.52, Life Crystals can now be crafted into Heart Lanterns with 4 Chains. In addition to being a light source, a Heart Lantern will provide a health regeneration buff to any characters standing within about 48 tiles of it. This regen effect stacks with that from the Campfire. Multiple Heart Lanterns won't increase the effect, but can cover a larger area, and are useful in the construction of boss arenas.