Just because it isn't checked, doesn't mean that you should not comply with the rules. Sailboat racing has a long tradition of being a self-policing sport. Being at the right weight is part of being class legal.

Guaranteed, the first regatta that you do really well - somebody will protest you for being underweight. If you knew you were underweight to begin with, it could also be a Rule 69 (Gross Misconduct) violation, which could get you banned for a year or two. For 5 lousy pounds.

Carry the weight.