Lester's absolutely right. Playing anything (including tuning) whilst others are performing is the height of rudeness, and is totally unneccessary. Would an unaccomperated singer start singing scales whilst another was performing, on the basis that they were warming their voice up? Of course not - good manners make it a complete no-no.

Everyone knows that stringed instruments need to be tuned before a performance, and that they need to be re-tuned (or tuning checked)during a performance, and audiences understand and expect it. The time to tune up is before any performances begin, with a quick check and tweak when you first go on - absolutely NOT while someone else is playing or singing.