So from a search perspective, WikiTree will search for LNAB as well as "Other Last Names" and "Current Last Name", so you probably don't have to worry much about that part.
However, I try my best to remain consistent, if that's feasible, but more than this I try to be accurate if I can. So if McPherson is the most likely LNAB for all children, then I'd use that. But in the "Other Last Names" field, I would put the variant (MacPherson). Or vice-versa if Mac is the proper and Mc is the less likely. The find feature is a bit flexible, so it might find them anyway without it but it's better this way as it covers you for both circumstances.
I'd also put something in the biography to describe your thoughts in which name you chose, and that way anyone backtracking your work can see your justification and add to your research.