Returning to the US from Rome, Italy. British Airlines to Heathrow in London and US Airways to Charlotte on same reservation. Plan to check bag all the way through but will I have to reclaim bag, go through customs and recheck? I believe I will arrive in terminal 5 and depart for US in terminal 3.
British Airways and US Airways are both part of the OneWorld alliance. As such, even if you were on two tickets, you'd be able to check your bags through. As it's all on one ticket, you will definitely be able to check them through (and that'll be the default). You'll hand over your bags in Rome, and won't see them again until Charlotte.
(Check the baggage tags you're given, they should show the bags going to Charlotte via Heathrow. The check-in staff will almost certainly confirm to you that the bags are checked through as well)
When you land in Heathrow, you need to follow the Purple flight connections signs, then Flight Connections T3. You'll head downstairs to the transfer area, wait for a bus, take a 10 minute bus ride past all the planes to Terminal 3, then go through security in T3. You won't go through immigration, nor customs, as you are remaining airside in international transit. There's quite a lot of good information on the process on the Heathrow airport website
Note that if you flight was Rome-Heathrow-Charlotte-elsewhere, no matter who the airline was, you'd need to collect your bags in Charlotte (as your first point of entry into the US), clear customs with them, and then re-check them for the US domestic leg.
Base on this post, BA has interline baggage agreements with UA and you mae 2 flights in 1 reserve. So you don't have to recheck baggage.