blogward has it right.   I sing in the shower, on the street-- (it's 4 long songs or 5 short songs to the subway),-- around the house, in the car, in stairwells at work, even walking down the corridor--in addition to music parties, in duets with Jan (and anybody else--she calls me a duet slut), and of course all my choral group's rehearsals and concerts.   Never have had a serious voice problem.   (I do think basses have easier than tenors--especially early in the morning.)

Our conductor warms us up with scales etc every time, but I find I'm permanently warmed up all the time.