Definitely go oversize on the trailer. The ONLY time you will regret not doing it is when your SB30 is lyiong on the interastate because the tires blew out from being overloaded.
The rule I use is to have at least a 10% safety factor from the absolute max weight.
Take the 4635 lbs for the boat, add fuel (@7.5 lbs/gal) plust trailer, gear etc. then add 10% and I think 7000 lbs is a good number.
Over spec your trailer and tow vehicle, you will never be sorry.