There are different answers according to the context. Firstly though it is worth pointing out that the part of the question "and still be in a stable, useful GEO orbit" is not so clear in terms of the distinction between "stable" and "usable".
One view with much merit is that nowhere in GEO is stable. GEO satellites are kept on station by manoeuvreing to oppose the disturbances. The following topics attempt to identify why not and what the typical usable bounds are.
It is typical that GEO satellites will be found within +-50km of the exact GEO point, with inclinations less than 0.1deg and longitudes within +-0.05 deg of their licence-assigned spot.
Luni-Solar (North-South) perturbations
All satellites are subject to this perturbation which causes the inclination of the orbit to evolve. Left unchallenged the inclination will rise to about 15 degrees and then back to 0 degrees in a 53 year cycle. Broadcast TV satellites mostly need to station-keep to 0 degrees +-0.05 so that ground antennas don't have to track. Manoeuvres can be anywhere from once a month to every few hours. Mobile telecommunications satellites can tolerate quite a bit of drift as the ground terminals aren't pointed at them in the first place. The limits in this case could be dopplershift or movement of the satellites beam footprint on the ground.
Earth tri-axility (East-West)
There are just two stable longitude points and all other places involve some drift. The effect on the satellite is that is orbital period changes and it moves slightly out of GEO and starts drifting in longitide. The same conditions for broadcast and mobile telephony satellites apply though ITU licence conditions generally also constraint satellites to +-0.05 degree, sometimes +-0.1 degree, slots to allow sharing of the spectrum.
Solar radiation pressure (Eccentricity control)
The shape of the satellite will determine how much it is affected by solar radiation pressure. The effect on the orbit is to change the eccentricity without changing the period. This causes both doppler shift and a daily East-West libration that is super-posed over the East-West drift. Usable orbits tend to have e = ~0.0005 or less and the manouevres are usually programmed to take place at the same time as East West manoeuvres.