I always keep mine pretty hard. I get a better ride in the rough with them hard as they don't move around so much. Damage, if you stuff the boat for example, is less likely too.
You have to watch them in the summer. I always let a little air out if the boats going to be sitting on the trailer in the sun or I know temperatures are going to rise. When on the water its much less of a problem as the water/wind keeps the tubes temperature down.
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