Since I don't have a clue whether or not the TXDR in the a/c I fly are fully transistored or have a valve somewhere then I switch to STBY after start.
I also don't know if it auto-STBY during code changes so I manually select STBY during code changes. That guards against inadvertant 7x00 alarm bells. It's also easier/faster to get the code set if it's entered in the order given, instead of of out-of-sequence as one person has suggested.
The argument about not using STBY when entering the code due to ATC issues falls apart if there are TXDRs that use STBY automatically.