The Sportster 1200s were ok for regular grade till 2004 when the compression was bumped to 9.5:1. I'm not certain about what the Big Twins ran for stock compression ratios. Since H-D engines have a rather large combustion chamber, fuel with too high of octane might actually reduce performance due to flame travel or lack thereof.
My buddy was running premium in his '99 XL1200C and on my advice he tried lower octanes. No difference in performance, and no pinging. Only difference is on a hot restart, it would sometimes backfire (older Sportys had dual-fire ignition) out the exhaust. Sometimes embarrassingly loud too. Only once in a great while would it do it on higher octane fuels.
If you're worried about running lower octane, try dropping it little by little and pay attention to how it performs. At 1/2 tank, fill it with midgrade for a final octane of 90 (assuming 91 octane premium), then next time you're at half a tank, fill it with 87 for a final octane of 88.5, etc. If you're to the point where you're running straight 87 with no pinging, stick with it. No sense blowing more money than you have to out the tailpipe, even though we're only talking at worst case about $1.50 difference per 5 gallon fill-up compared to premium grade.