Sherman troops in Normandy would smother a Tiger or Panther tank with HE rounds, as they had no faith in their AP shells. Time and time again, this tactic worked well, immobilizing the heavier German tanks, smashing optics, and sometimes causing spalling on the inside of the vehicle. All of which tended to make the crew bail out, after which they were machine gunned.

Does your game represent this? or are Shermans with the 75mm generally unable to deal with Tigers or Panthers from the front?
From reading "Tank Action" (albeit this was a British Sherman troop - not sure if tactic was used by the US or Canadians), it seems to be their main way of dealing with heavier armor, right up until 1945. Fireflies did help, but they were a little ungainly, and had to strap their gun down and face backwards when moving long distance - they were generally employed as overwatch or as a reserve brought up when the Shermans encountered a heavy target.
Another interesting tactic from this book is commanders dismounting and walking their tanks through difficult terrain and scouting out enemies, although this was often fatal for the commander...