Just my point of view. I have been using my present detectors for over 12 years with the same coils and NO coil covers. I keep my coil about 1" above the ground while scanning. Now before you say it, let me mention that if you need that extra inch, you need a better detector. Also scrubbing the ground can cause false signals and so can dirt getting between the cover and the coil which is highly possible if you are scrubbing the ground. Frank...
I suppose with some detectors it could be a problem but in over 4 decades of scrubbing the ground, I've never run across the possible problems you've described with the 11 detectors I've owned. I guess it would be best to run some test first to see.