Should I ever not use a Life Crystal?

Solution 1:

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.

Solution 2:

If you have the maximum health, 400 hp, then using more Life Crystals gathered from Crystal Hearts will have no effect.

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!