This one is tricky. Both ideas have ups and downs. If you can cap the tank that helps a lot when actually travelling. If you have to manually dump it you'd have to do so before most jumps if you weren't carefully watching the exact fuel tank amount. On the other hand, there are actually advantages to being able to manually dump too. For instance, if you normally keep it full that would let you dump enough to reach the optimal mass for a slightly better jump (perhaps especially handy when you realize you're stuck among a bunch of brown dwarfs and won't reach a fuel star in time without going off course.) I guess in a perfect world both would be an option. EDIT: As I think about it, perhaps both is overcomplicating it. You CAN put in a smaller fuel tank after all. But there's no true mechanism to dump at all. (The closest you can do is turn everything on, deploy hardpoints, and go nuts for a bit to try to burn it off the hard and slow way. It could potentially take hours in some cases...)
Actually, I still don't understand why you can't dump fuel. In fact, it still seems really strange to me that the only way to share fuel is via limpets + a dedicated controller. I feel like you should be able to dump a canister and someone manually pick it up just like anything else. As with cargo pickup limpets, the whole point of them is to add convenience, increase efficiency, and decrease the time required for dealing with it (especially handy if you do a lot such as with the Fuel Rats.) It isn't as if pure hydrogen is more volatile to store than many of the raw elements and compounds we can carry in cargo and dump at will. (Heck, there are literal explosives to be carried sometimes... Now that's what should be scary to carelessly dump.)
