I used it only when needed.

Depends what sort you are using too.

Petroleum based "barrier" creams or
Non-Petroleum based rub in salves.

i would feel safer using the salves for long term use on my child.

the petroleum based "barrier" creams also make MCN ineffective (unless you have a liner in, to take up the cream), so that's another reason i avoided the barrier creams.
We use the Gaia Soothing Lotion. We're a MCN family, but use microfleece liners to protect the nappies. We also use cloth wipes rather than disposable wipes as they are far more gentle on sensitive skin, not to mention cheaper and eco friendly.

Interesting to see that there's a mixture of use, some every change, some only when needed and some now and then. But, all methods seem to be effective.