I do seal my gaps but I do both sides when I cover the plane so it's one piece. You only need to seal the bottom. Unless you have some big gaps you really will notice no difference in the way your plane fly's. I do it out of habit and it helps me at clean up time at the end of the day, that's about it!
Oil gets under tape so if your plane is already covered clear covering is the best thing to use. Remove the control rods, bend the control as much as can then iron it on one side first then iron it into the groove, lift the control closed and do the other side so the covering is ironed smooth and flat. Shazam, hinge gap is sealed.