You never have to pull the assy off the caliper.
I cleaned the part of my pistons that were exposed until they sparkled, so that I wouldn't be pushing the dirty
pistons back up into the calipers when I put on the new (thus thicker) pads. I had the assembly off the bike
anyhow, so I used compressed air to blow the pistons further out... the seals don't start right at the edge
of the caliper piston holes, look how far back the dirt goes beyond just the part of the piston that is
exposed:

After I saw this, I popped the pistons the rest of the way out and cleaned them, lubed them with
brake fluid and pushed them back in.
I'm pretty sure that I'm going to do all this every time I change brake pads from now on.
-Mike