I've been competing with USAPL long enough now that the toes-only bench is REALLY awkward for me. I find I get a better leg drive with my feet flat now. My arch hasn't suffered for it, either. Incidentally, I've become much more flexible.
On occasion, normally after some pretty heavy squats or deads earlier in the week, my hips will be really tight and if I don't do a good job with mobility drills before benching, my feet will come up at the outside edges. This is easily remedied by doing a better warm-up and mobilizing my hip flexors before getting on the bench.