The question of when, and whether to raise is a strategic decision unrelated to the stage of the hand you're playing. You can face a decision to raise at any point during a hand, from that pair of kings you've been dealt before the flop, to that straight you've made on the turn or river. And while there are exceptions to every rule, let's see if we can build a strategy that is correct for most situations you're likely to encounter at the table. Raising is one of poker's eternal enigmas.
Some players love to fire in raise after raise.
Others never raise unless they're holding the nuts, and even then its done with trepidation. I've seen some players who will raise from early position with a hand like As-8s, believing that any suited ace is a powerhouse. Some players always raise with Big Slick. Others, far fewer in number, treat A-K like anyother drawing hand, and call with it. Some players will raise with any pair of 7's or bigger, and they'll do it from any position, regardless of the texture of the game.