I have a set of trailerable hull covers that came from Salty Dog Marine (I can't remember the brand on the covers .... arrrggg). They take a bit more time to put on but they're priceless for keeping the hulls clean when trailering (I do about 12,000 miles a year with the boat behind my RV). I've towed with them and without them so I can tell you that as far as grime goes, if you are going to cover anything, cover everything. If you are only looking for some chip and ding protection (and you are towing with a narrow vehicle) some basic covers over the bows might be all you need.

As far as the trampoline, I never take it off for trailering - it's horizontal so I don't imagine it makes much of an impact to my already 'less-than-desireable' fuel economy. Removing the trampline also involves loosening the beams and removing one hull from one forward beam on my Nacra hence it is less practical than on the H20 or 16. I've never felt that the trampoline was overly dirty from towing.