It depends on the flies you're using and how good your casting style is. A level leader of that length with a single heavy-ish fly (read as a gold head or well weighted nymph or lure) on the point should turn over ok in just about everything but the most extreme of windy conditions, even with a lightweight dropper. A tiny nymph on point and another on the dropper, or if you're using a particularly unaerodynamic patterns and you'll probably struggle with a level leader and need to add some taper to the cast.
I can't think of a single instance where having a tapered leader would harm your presentation. That's not to say there isn't one, it's just I don't know what it would be
I'm quite certain too, that there are people who can cast a level leader of 15 or 20 feet using unweighted flies in the stiffest of oncoming breezes. Sadly I'm not one of them