The lab clears basically all salaried employees at L or Q; if there's any chance that you might look at classified data some day (IE: if you want to interview at another group internally, or have some useful expertise that can be leveraged somewhere else), it saves everyone time and energy if you're already a Q. There's some financial reasons not to do it, but I think the math works out in favor of not running multiple levels of background check on every new hire (if they were to give you an L and then a Q).