Not even sure this is supported by the Mac version but I think it'd be great if SourceTree would allow marking/unmarking files as --assume-unchanged and listing them.

I use aliases for command line to do this as I need it on regular basis:

[alias]
    ignore-on-commit = update-index --assume-unchanged
    stop-ignore-on-commit = update-index --no-assume-unchanged
    show-ignore-on-commit = !git ls-files -v | grep "^[[:lower:]]"