If the tire is 90% on then the bead should be seated enough to allow the tire to hold air and seat with added pressure. I used to have this problem years ago when I was still using soap as a lube. It could take close to 100psi before some tires would pop on. Now using nomar lube, all tires pop on with under 20psi, even the E07 Dakars I put on a few weeks back. When you have a situation like cycledude mentioned, where the tire is too narrow to even begin to seat is where the ratchet straps, lighter fluid, bead blaster techniques may be needed.
I just mounted 4 truck tires 235/85-16 on stock 6.5" wheels and I swear the tires were 3" wide at the beads. Tying to get to seat on 6.5" wheels took some creative measures. Motorcycles are easy normally, but must use correct lube, not soap.