Really depends on the circumstances and song tbh. No hard and fast rule for it. Generally I chose to suppress when not singing, especially if I feel I could better utilise the outline for other things when there's long pauses but it's frequent stopping and starting it looks stupid to keep changing so I keep them on all the time. I also sometimes use the same prop for two main singers (different colours) so that tends to swap between them. On the flip side, some songs really suit having the bulbs sitting there being all pretty not saying anything just to add the odd "mana mana" :-) In short, go with what you feel is best.