I would say a good practice with folds (and maybe in Haskell in general) is that either all be strict or all be lazy.
In the expression: foldXX f init list:
Remember that foldr does:
x `f` ( ... the accumulator ... )
and foldl:
(... the accumulator ...) `f` x
The accumulator has to match a non-strict argument of f, so that f will be able to return results even if the accumulator is not fully evaluated.
More detailed:
if f is lazy in its second argument, then use foldr. Everything is lazy, you build a very small thunk since nothing is evaluated.
In the rare cases where f is (also) lazy in its first argument, you can use foldl.
And of course, if f is strict in both its arguments, then use foldl'. Everything is then strict, you build no thunk.