Unless you're shooting at great distances and want every little advantage...no, they really don't make a difference, especially if you have a good upper. I would imagine they would have some accuracy value if you plan on having Batman's utility belt mounted on your rifle. I must add the disclaimer that I prefer "stock" rifles over tricked out ones...so my answer may be a touch biased.
A freefloat upper will remove the possibility of an accuracy shift if you're using cover. Resting the rifle on a barricade, etc.
If you're shooting irons at close range, yeah, probably won't notice the difference. Shooting a variable scope or an ACOG at 200-500m? Yes, you WILL notice a difference if you're a decent shooter.
With the advent of easily mountable lights for traditional handguards and the release of Magpul MOE handguards, no there's not a ton a ton of reasons to use a free float handguard if....
- you don't use a modern weak arm far forward stance
- you're not concerned about long range accuracy (say, past 200 yards).
With pressure on the handguards, one can easily create 5 MOA of deflection.