You can apply Vaseline or diaper rash cream to your baby or toddlers bottom and diaper area every diaper change if you want too.
But I only applied diaper rash cream or Vaseline to my kids bottoms when I changed their diaper for the night when they would be in wet diapers longer than normal.
In the day they got a diaper change every 1 hour to 2 hours and then as they got older as in 2 and 3 years old the kids got diaper changes every 3 hours because they wet less often.
I would check their diapers every 2 hours or so but usually they were not wet enough to need changed yet so usually every 3 hours was good enough to when they needed a new diaper unless they pooped.
If we were on a road trip I would put some Vaseline or diaper rash cream on their bottoms because we could not always stop to change their diaper especially if we were traveling at night when they were sleeping.
As long as you change the child's diaper often enough they should not get a rash and make sure you wipe after every pee as well.