The reasons for a tight installation is for ultimate mechanical strength and to prevent a fuel leak as much as bonding. Also the finished installation is cap sealed. Any PD between the fastener the skin and the collar/nut is fully enclosed.
On the 787, the tight fit is required as part of the lightning strike protection...
A slight gap between a wing-skin fastener and the hole it goes into could be a source of sparking as current jumps the gap. Boeing will install each fastener precisely and seal it on the inside to ensure a snug, spark-free fit.
I would be very concerned about the isolation of the grounding system. This was one of the problems noted with the battery issue on the aircraft already....