Urethane or not rough pine walls

You can leave them alone, or you can coat them.

If you want "the natural look" leave them alone.

Any sort of varnish on rough-sawn boards is going to look very out of place.

I'm guessing the pine is being installed on the interior of the garage? Although it can (and commonly is) used as an exterior wall. Either location Board and Battan walls are rustic and natural.

If used as interior walls they can be left unsealed. Rough Pine will eventually become less bright with age. This can be prevented by applying a sealer to protect the wood. A urethane will work, but if used on rough lumber it may not look aesthetically appealing with any sheen, but matte or flat (this is a personal call).

A water based floor sealer will be he easiest to work with. There are also deck sealers that can be applied (Flod brand gives good results as does Thompson). All will dry clear to leave wood grain viewable, but protected.

It will be easiest to apply whatever you ultimately choose before installing the B+B pine. Several coats all sides is ideal.

The same principals should be used if boards will go on exterior wall. Only you must install a moisture barrier (30 # felt paper or Tyvek) under the boars first.