Hi,
I would recommend steeping everything. Mints and fruits are fine to shake and vape, imho, but once you find a vape you really like, make 250ml or whatever large amount you can mix to and then when you get down to about 50ml left in the bottle mix up a fresh batch and compare the two. If you put dates on your mixes you can see the difference that steeping has made to that juice and decide then if its worth it.
I have found that nothing beats time for improving juices for just about every flavour including mint. I have three adv's, one is mint, I have 250ml of mint which I made 06/10/17 with about 30ml of older stuff left before I open it. There is definitely a difference by being able to leave that mint for at least a month before I start it and it will be a couple of months old before I finish it.
Custards are the same but I think there is a balancing act for steeping with custard, the longer you leave it the more caramel it gets which then is down to personal choice, I like my custards to tastes like a blend of vanila and caramel so I try not to leave them more than two months.
If you like your lime with a hit try it with 2% Inawera Menthol. I make a version of this and its very popular.