TLDR: Yes, 5mg, but no further, provides additional hair benefits with very low risk of any serious or lasting sides.
Like most BP meds, there's an initial period where the vessels targeted by the med can over dilate, leading to possible side effects, which typically resolve within a few weeks to a month, because the body rebalances by making specific vessels tighten back down.
Minox 5mg is a "child" dose, well below the usual dose for adult BP treatment, but it is possible for the most common side, water retention/swelling to occur. If this even happens (unlikely), it should clear up quickly.
Studies show 5mg does provide significant hair growth benefits over 2.5mg. I consulted with a top hair replacement surgeon and he was using 5mg/day.
Beyond this dose there don't seem to be any additional benefits, Once a day is fine, though if sides are troublesome, dividing into 2x a day can lessen them,
The only other issue is body hair growth accelerates, but it's a lot easier to shave your back once in a while than it is to grow more hair on your head via some other method.
Regular use of MT-II has reverted greys to black. Not brown, not blond, not any other color, black.
It seems to bring melanocytes in hair follicles back to life, but like most resurrected zombies, they come back not necessarily looking the same as before. I'm dark brown so it's fine for me. One you stop, they'll revert to grey within 6 months or so.