In general, who should do more work: popFront or front?

[surlymoor via Digitalmars-d-learn]

All my custom range types perform all their meaningful work in 
their respective popFront methods, in addition to its expected 
source data iteration duties. The reason I do this is because I 
swear I read in a github discussion that front is expected to be 
O(1), and the only way I can think to achieve this is to stash 
the front element of a range in a private field; popFront would 
thus also set this field to a new value upon every call, and 
front would forward to it. (Or front would be the cache itself.)
At the moment, I feel that as long as the stashed front element 
isn't too "big" (For some definition of big, I guess.), that 
built-in caching should be fine. But is this acceptable? What's 
the best practice for determining which range member should 
perform what work? (Other than iterating, of course.)