i have gathered from these forums that applying for EAD renewal 90 days before its expiry is a good idea. the reason for that is, if your EAD renewal application has been pending for 90 days and you still have not received it, then you are entitled for a walk-in renewal at your local INS field office. In case 90 days have not passed, they will not entertain walk-ins, and if you haven't received your EAD (and you were working on that basis), then you are out of luck.
BTW, the starting effectivity date on the new EAD is the date of its approval, *not* the date of expiry of the last one (as these documents are independent of each other). so a gap in their effectivity dates is not a desirable thing.
hope this helps.