Mirror slap is an intrinsic vibration source (internal to the camera), and it occurs just as the exposure is being made. Even though a good (heavy) tripod can help damp that a little bit, it's intrinsic - right next to the sensor. My interpretation of that is to always use mirror lockup when on a tripod, unless you're shooting action that precludes it. I'd say the only exception to that is with really long exposures (several seconds), where the duration of the vibration is such a tiny fraction of the total exposure duration that any vibration-induced blur will just be averaged out (and even then, with an exposure that long, what's the point in 'saving' another couple of seconds by not using MLU with the 2-second timer?).