It depends on the tank. Some are filled from the bottom, so you have to take it off. Some are top-fill, so you can leave it on the mod while refilling. The main thing to consider is, how neat an operation is top-filling, for you? They say that kayfuns can be top-filled, by covering the airhole, turning it upside down and sideways and then back to right-side up, saying a magic incantation and holding your mouth right, but all I ever managed to do when I tried to top-fill a kayfun was waste 3ml of juice. Fortunately it wasn't on a mod at the time.

But if you can top-fill and not make a mess, then leaving it on the mod is better, because the more times you screw/unscrew the threads, the shorter their lifespan. One of the things I love about my Achilles is that it's top-filled, and since it's an RDA there are none of those pesky "vacuum" issues that make top-filling a kayfun so perilous (and messy).
Andria