In a very small percentage of people this can happen, probably most often in the front. If you're not desperate for quickest re-growth at the moment, I'd start out with 2% in order to minimize chances of this. Best to ease the follicles in slowly.
If you use Fibres and Derm concealers, after applying, I'm guessing you need to wash hair every day before you apply minoxidil, what's the best regimen for this for you guys?