Never use side where an equally good and sure result can be obtained without it.
Far more important than the use of side are straight and central cueing with well-judged strength and ball contact.
Side is only valuable in strokes that cannot be made so well or at all without it, and these are comparatively few.
The three main cue contacts are centre, above centre, and below centre. The three main ball contacts are half-ball, three-quarter-ball, and quarter-ball. By the use of these contacts, coupled with judicious strength, a grand game can be played.
Indeed, many a hundred break has been made without the use of side at all.
Side should be avoided both by reason of its complexity and of its uncertain effect on the cue ball, even after the stroke has been effected.
Take the case of a cannon made with a lot of side, which might have been just as well made with a plain stroke or with a mere trifle of side. The cue ball, before coming to rest, takes a weird course that makes its ultimate resting place largely a matter of chance.
Another point in favour of the plain stroke in cannon play is that the cue ball can not only be better guided, but the proper strength can be much better applied. The mind of the striker is free to consider and deal with the really important points.
Even in pocket play the use of side is often more dangerous than beneficial, and this is especially the case where what is called pocket-side is not employed. Side helps a ball that reaches the open shoulder "running," but is disastrous when the reverse has been employed.
This is especially the case with fine strokes, which should, consequently, under such circumstances, either be made without side or with running side.
The prime uses of side are: (1) To make blind pocket play easier; (2) to obtain different results through the combined agency of a slowly-moving cue ball and the nap of the cloth, and (3) to secure a different course from normal for the cue ball after it has come in contact with a cushion.
Where these results are needed side should be employed.
If neither of them is necessary it is better let alone.