I think evman said it the best. If you have extra outs besides the flush, it's a play you need to make sometimes. I've thought a lot about this, and in Pot Limit No Limit by Rueben and Ciaffone (spelling?) they deal with this topic.

Their take is if you have an overcard that's hit...say a king, or the Ax for the nut flush, you might push given that maybe your top pair will hold up.

A gutshot straight draw will give you 13, but at the $25 tables I play on, if you are in early position and are not drawing to the nut flush, well...it seems like a case of higher limits chasing less and lower buy in's chasing more. Your giving that Ax good odds to chase too.

This is just my (actually their) 2 cents and I haven't been playing long enough to be like... a reliable source, but I think this would also really depend on if you were at a full ring table or six max.

My basic guidelines are:

If you had the extra outs of top pair, it's time to decide if you want to gamble. If you have a gutshot straight draw, It would depend on your read of the guy. If you had the additional outs of a open ended straight draw you should push everytime. If your just looking at the non nut flush draw with no over cards (nine outs) in early position you should muck it.