My concern with rogaine is not always just the alcohol it's the acids that they also use to help with the absorption of the medicine. It can be very irritating to the scalp. We know that any irritations to the skin that might include dryness, inflammation, redness, and or swelling can speed up a fading process if used during healing. It technically takes about 90 days for the basement membrane to close (thin layer between the epidermis and dermis) and until that happens pigment can still escape out of the dermis and be exfoliated through the epidermis.
Another thing is that going on and off rogaine during the smp treatment process is not good for you. It can cause a little shock loss. The hair will return but it's not pleasant and it's not a service to yourself.
I would hold off until you are 90 days post op. After that any small amount of fading that it might cause does not outweigh the benefits of the med.
Hope that makes sense and helps.