we only used sudocream/diaper rash cream when we thought it was needed (if she was red etc.) But when she was little we did put vaseline on her bum for every diaper change. It formed a barrier and also made cleaning the poo diapers easier
Sudocrem can be applied at every nappy change, but please remember to only apply a thin layer as a little Sudocrem goes a long, long way.
my lo is 21 months and i have only just run out of that tiny little free pot they give you in the bounty bag when lo is born , lol. ( and he emptied half of that out a few months ago sticking his fingers in it) as he has never had a severe nappy rash, i just use a tiny bit if he starts to looks red and it has usually cleared by the next change