When I first bought GW36 years ago, my group misread the rules and played the first dozen games with “full access to allies bases”. Since then we’ve played both “full access” house rule and vanilla rules, without significant balance issues.
With full access, USA shuttles troops to UK quicker. But the USA lands somewhere in Europe anyway and therefore full access doesn’t affect anything.
Eventually we settled on “Full Access to Allies Bases” as a house rule, and we like it.
It’s more historical, for the navy it doesn’t take the American fleet a full year to get back to America. For air forces…we figure that even if historically the Commonwealth and the USA didn’t share air bases; we assume Normandy had more then 1 air base, so in game everyone benefits.