I don't think there's a strict rule, but I always use regular conditioner in the shower and then add more leave-in conditioner depending on how my hair feels after the shower. If you're really worried about dry hair then I'd say that's probably the best way to go, that way you don't have to use a ton of leave-in to have soft hair. And if you're worried about volume (my issue) then don't put conditioner on your roots, just your ends. Hope this helps!

I don't wash everyday so I use both and it keeps my hair soft and managable.

Also, make sure you don't apply conditioner directly to your roots. (I've heard it makes your hair fall out... truth or myth? You tell me.) You should just be applying it to the bottom half of your hair, which is where most of the damage is anyway ;)