The blank tile is the most valuable piece in the game because it unlocks any word. But it scores 0 points, so use it wisely.
Save it for a bingo
A blank that enables a 50-point bingo is almost always worth more than spending it on a small play.
Don't waste it on common letters
If you can make the word with a real tile, keep the blank. Use it for hard letters or to complete long words.
Mind the endgame
Keeping the blank at the end costs no points (it's worth 0), but you lose its potential. Balance risk and opportunity.