I certainly don't know if that's truly the reason, but it makes at least "some" sense. However, you might be better off spraying the rollers when you get home - before the salt has a good chance to setup real well. Spraying them before you leave can't hurt, either. Grease is great - like you mentioned - but obviously the rollers are an awkward thing to grease on a regular basis. Maybe some spray lube (silicone, for example) on a regular basis? Just tossing out some ideas.
Here's two more...
-- If the rig is stored with it's aft end near something sturdy, before you go the ramp, do a quick tie-off from the stern eyes to that sturdy thing. Loosen the winch a turn or two (and stern straps), drive forward. Untie and re-tighten winch/straps. Seems like a big process, but it could probably be done in about 5 minutes.
-- On your street or the ramp parking lot (somewhere where there aren't any cars right nearby), loosen the winch strap/cable a turn or two (and the stern straps). Get going in reverse and slam on the brakes. You can do the same thing in forward to move the boat back to the bow roller before re-securing for trailering. Although if you do it at the ramp, you don't need to worry about it. Before anyone flames me for this, I've done this so many times (for various reasons) with boats both smaller and much bigger it's almost become a normal thing to do. It sounds crazy, but it's actually pretty easy and safe. BUT, as a pre-caution... do not remove the safety chain!