If you do use a long post, make sure it's the prime suspect if you get any mystery creaks.

Last season, after nearly 3 months of a constant ticking noise with every pedal stroke, I finally figured out that it was the long seatpost, inserted beyond the point where it was a tight fit in the seat tube, that was causing it. I had previously replaced a Thomson setback with a zero-setback version, but I used a longer one that I found on eBay. I took the drivetrain completely apart twice before I figured out the culprit. I cut the seatpost down and it's been great ever since. Since I didn't have a saw guide, I used an old seatpost clamp and an old stem (that I did not care about) on either side, clamped the stem in a vise and cut between the stem and clamp. Cut was near-perfect with a fine-tooth sharp hacksaw blade. I filed a nice chamfer into the edge and took off any remaining sharp edges with some emery cloth, and if it weren't for the anodized finish coming off at the cut, you'd never know it wasn't from the factory. It also helps that I have an engraver to make a new min. insertion mark and to note the size, but as long as it stays on your bike you're probably okay.