On my last boat I had the spare tire mount that clamped to the trailer frame up by the bow. It held the tire very nicely but if the launch ramp was steep it was possible for the boat to hit the tire as you loaded it, which left black marks on the hull. Tire was also in the way for washing/waxing.
On my current setup I have a spare tire carrier in the back, underneath the trailer, tucked in between the prop guard and the fender step. That definately puts the tire out of harms way and there is no way for the boat to bump it. The only downside is that the lock gets dunked everytime I launch which makes me worry that it won't work when I need it to. I operate the lock every month or so to keep it working.