> Is it fair to assume most mouses have a scroll wheel?

Probably not, a lot of musicians develop on the go (planes etc) so they're dealing with built-in trackpads pretty often. You can still scroll but it's not as ergonomic.

This is one of the things which helped sell me on Thinkpads with their three physical trackpad buttons and trackpoint, middle click+trackpoint gets you your scroll wheel and it is quite ergonomic.