You do run the risk of tube damage with under inflation, many moons ago I pumped up a RIB, felt quite firm by hand and I was quite happy with the pressure - out of curiosity I checked with a gauge and the pressure was just over 2 psi, RIB manufacture stated a tube pressure of 3 psi or you ran the risk of issues - for the sake of a few seconds I now religiously check with a gauge

Oh, chatting to a boat dealer a few years ago, heading to france early one morning in a 8mtr, he knew the tubes were a little soft but felt as soon as the sun came up and the pressure increased he'd be fine - one of the rear cone seams failed and he limped back to the south coast