Once the sides have been EQ’d to taste, you may now control the dynamic range. Not very many transients exist in strictly the sides unless hard panning or stereo imaging has been applied in production. Levelling the sides with a compressor can help define the stereo image of the track by setting the ambience and feeling of the stereo sounds.
Having transients exist strictly in the sides can be quite distracting and take your focus off the central image of the track, then again, some of the best sounds and feelings can come from creative hard panning. In any case, a degree of dynamic control should be maintained to apply a sense of cohesiveness. As in all stages of mastering, apply compression subtly here as to not mask the main elements at focus in the mids.
In terms of compression on the mids, multiband compression is to follow in which case the mids and sides will be dealt with together. Bear this in mind in case you wish to dynamically control mid information as a pre-stage to multiband compression.