Join Date: Mar 2009
Location: Missoula, MT
Bikes: Giant TCX 1 touring conversion
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A *slightly* lower than optimal saddle height helps with dismounts and remounts, and won't hurt your pedaling dynamics too much in a racing situation where you are going to be all over the saddle, out of the saddle, off of the bike, back on the bike, rinsed with mud and repeat... it just doesn't make as much difference. By contrast, you'll feel every millimeter of your saddle being too low on a long seated road ride.
The more upright position is an oversimplification, but a convenient one. The biggest thing is weight distribution rearward to deal with offroad obstacles, steep descents, etc... to keep from going OTB and to be able to shift weight around without disturbing the steering too much.
Don't buy into the old garbage about sizing down for 'cross. At your height, you will need every millimeter of headtube you can get. I'm 6'2" on a 60cm frame, and I run my bars just shy of saddle height, so that means lots of spacers and a re-evaluation of stem length once you get it high enough. It looks funny, but it works great.
Then again, I'm no expert, and I'm sure lots of guys will reassure you about how you can race cross in a flat-back aero tuck.