The default rounding up method for my native currency is to round up to the nearest 10 monetary units. What I like most about this, apart from setting a small amount aside every time I use the card, is that the balance on the main account always looks neat and clean with amounts that are divisible by 10.
The current accelerators are “×2 ×3 ×4 ×5 ×10” and they have one thing in common: They introduce decimal withdrawals so that the main balance more often than not uses decimal numbers, which triggers my OCD.
What I’d rather like to see than multiples of the round up sum, is rounding up to, say, 20, 30, 40, 50, and 100 monetary units. This will accelerate and keep the main balance tidy.