I see it all the time as a Choir Leader. Members might roll their eyes or even mime sometimes! I get it - I really do. But warming up is really just a kindness to yourself - and you deserve that don't you?!
If you were going on a run - you'd warm up! You wouldn't start a football match cold - you might hurt yourself and the same is true of the voice. If I could think of the worst scenario I'd even suggest a glass of iced water before NOT warming up too! Are you getting my point? Drink warm or room temperature water and just start with some lip trills or hums. Your voice will thank you. Your rehearsals will be more satisfying and your voice will behave as you want it to (most of the time and with effort!).
I have an album of warm-ups that's going to be on sale soon - think of it as a kindness to yourself ;)