Use it when you're somewhat far away from an enemy. Think of it as the opposite of Splash Wall, it's really good at range but is very easy to counterplay when it's close. BB is kind of useless close range because enemies can just walk into it but it is very good at protecting you when they can't. Roller is kind of screwed in this aspect (More reason to use Krak-On), but every other weapon with it has the range to fight inside of it while keeping their distance. Using it in high ground is also good because it makes it harder for opponents to hit the weak spot on the top. It also acts as a free Beacon during and after its duration so that's pretty cool.
I'd say for roller it actually
does want to place it as far as it can, and I think mint will too.
Since no one wants to enter a roller's bubble, as long as it's up, you're safe in your position, so your team can (hopefully) catch up to you and take a lot of space in one go.
As for positions, try to get in an important area for the objective, and prevent the enemies from standing in it. As for important positions, go where the enemies need to stand to play the objective, in front of the zone, the rainmaker, under both clam baskets etc.
You can also use bubble as a stalling tool, in a neutral state, if two people are taking time shredding your bubble, they're wasting time in an unwinnable 2v1, so your team's playing a 3v2 for a bit, it kind of works like a tent shield in that regard, you just want to ensure that you live after that, or at least trade, since it's not guaranteed that your team will be able to play off of that.
And one more note on vroller specifically, sometimes if your team isn't able to catch up to you for some reason and your bubble gets shredded, it's usually better to jump back to a teammate since you most likely don't have any paint around you to work with.