Should I apply 5% Minoxidil only to the areas very affected by thinning (i.e.: temples)? Or the whole scalp?

I've managed to halt my hairloss for years with 5-ARIs (Finasteride) but I think it's time to add Minoxidil to the regimen. I'm a NW2 progressing to a NW2.5 on my Temples/Forelock. While I do have some thinning in the vertex, it's very minor and it's hard to detect even on the microscopic analysis Should I apply Minoxidil only to area where the thinning is evident (temples)? Or should I also apply some of it on the vertex as a preventive measure? Thanks