It kinda depends. I think for newer players it is best to stick to normal cubelets. Since you don't have a lot of items yet, you don't get a lot of duplicate common/uncommon items. You can open a lot of them so you start building a collection quicker.
After a while, you can consider opening super/uber cubelets.
Super cubelets have a nice balance to them in terms of rarity/price. You have to get somewhat lucky to get good items out of them, but the chances of getting boring items out of them is slimmer, making it a good item to open when you have a nice collection of armour/heads and are after the better items, but can still benefit from getting more items over good items.
Uber cubelets have a very small chance of getting a head/armour, so open them when you want to miniatures/cages/win effects/gadgets etc. they take quite some time to get so only open them when you have an item set you already really like, but want rarer items to show off with.
I personally like to mix it up. I save for a uber cubelet and after opening it, I open normal cubelets for a while. Sometimes when playing a few games and obtaining multiple cubelets, I decide to save for a super. After a bit, I start saving for my uber again.
My Gf has a weakness for the excitement of opening cubelets so she opens all of them immediately, but saves her lootpoints she gets by voting for the occasional super/uber cubelet.