Quote: "So strings are more memory-efficient than integers only if your numbers are less than 3 digits (and maybe not even then, since your computer can't allocate things smaller than a certain size and will probably use 4 bytes anyway)"
And even then, using words or bytes would be even more efficient. I'm not sure how strings are implemented in DBP, but there either has to be an additional value (which again would probably be a word or dword) storing its length, or strings are 0-terminated, i.e. there's one more byte that's not part of the actual string that simply has the meaning "this is where the string is over".
So, if it's just about storing numbers, integers are most certainly always preferable to strings. The only case I know of where converting integers to string and vice versa is for GUI related stuff, e.g. if the user has to insert a number or if a game's interface shows a number in textual form.
Edit: Also, you might want to use strings or some other system when handling numbers that exceed the 4 byte limitations - but in that case, there's no point in converting the number to int, so I guess that's not the case here.