Well, as you've already realized, there is basically no perfect solution whether or when slice timing correction should be performed. This is some guideline I've seen:

If TR is relatively small (<= 2 seconds, for instance), slice timing correction is probably unnecessary since the acquisition time between two consecutive volumes is negligible. If TR > 2 sec and head motion is significant, try head motion correction first and then slice timing correction. If TR > 2 sec and head motion is small, go with the reverse order. If head motion is unknown, run slice timing correction first for interleaved scanning and go with the other way around for sequential images.

Gang