Follow these two opposing instructions in sound checks.
First, sing consistently like you’ll sing in concert. Don’t mark, because the sound person can’t get a reliable level if you’re marking.
Second, don’t oversing. You’ll be singing a lot in a dress rehearsal/sound check, and you don’t want to overuse your voice before the concert.
Of course, they aren’t really opposing. If you use a microphone properly, you never have to sing so loud that you’ll overtax your voice. A well-trained singer with a microphone has stamina to spare.