According to the Dow theory (which you can observe in every financial instrument), markets move almost NEVER in one straight line up or down, instead every time in a trend-move followed by a (weaker) counter-move.
When you're entering the market is up to you, but as I'm using leveraged positions, I've got to place a very tight SL. Entering on the primary move would kick my position out as soon as the pullback sets in.
I agree that markets seldom move in a straight line one way for long, but the downside of waiting for any kind of reversal is that you can miss the best part of the move, and also find that the 'pullback' might actually be a reversal. So you can place a closer stop loss, but with more chance of hitting it.