I've gone on single overnight trips where I didn't take a tarp, but knew it wasn't going to rain. And of course I've camped many times without actually using the tarp, despite having it with me.
“I think that when the lies are all told and forgot the truth will be there yet. It dont move about from place to place and it dont change from time to time. You cant corrupt it any more than you can salt salt.” - Cormac McCarthy