if they are rusted the seam under the seam sealer in the gutter is probably rusted too. So if you plan on drip checking the drip rail then have a close look at that first. Is it rusty? Will digging out the seam sealer and cleaning it up be a lot easier with the moulding off?
I'm currently working at a snail's pace on a 65 Mustang, I do this work for a living, and have no plans on removing the drip rail mouldings. It's not a full restoration and the mouldings are pretty clean so I taped them off. Sure I can take them off but it's more work and I'd rather save that energy for the work I get paid to do. Then again, drip rail mouldings aren't very hard, but you have to know how much little force it can take to distort them. Then again, I recently saw the shop guru take one off recently with a small slide hammer and it came off with no damage.