I rode my 75' cb750 around for a month with no fender and I didn't see any problem. In fact the oly time I ever got close to a tank slapper was with the fender on doing 80 on the jersey tpke. There are plenty of bikes out there that run fibreglass fenders, which dont' offer any support, and they don't have a problem.
If you are nervous about it a fork brace (or tweak bar in chopper lingo) will definatly put your mind at ease. Unless you are racing, ride on the street aggressively, or have a pot hole problem where you live I think you could make do without it, but remember if you have on fender and clipons or clubmans you will get a mudstripe up your helmet. I got caught in the rain once with my cb750 without a front fender (and clubmans) and the rooster tail off the front tire was so bad I couldn't see out of my goggles, I had to pull over and wait out the storm.