Seasonal changes can cause wood to expand and contract, much like doors or windows in a house or the neck on your bass. This can result in loose screws. So it doesn't hurt to check things once a season. It depends on what the climate is like and how it changes where you live.
It is important that speakers be tightened down so that there is an even pressure applied to the speaker frame. Tighten the screws (or nuts, depending on the construction) in a cross or star pattern. Like tightening a drum head. It is also important not to over tighten. This can warp the speaker frame. Some guidelines are provided here