Leave any major pruning until end of winter. If you are talking about the popular Spiraea japonica cultivars these actually are best hard pruned every year, once established. Whereas the other two, not so much.

If now it the only time you have and the shrubs are really overgrown, then do it. I agree There are better times to prune (as suggested above) . I would not prune more than 1/3 of the branches of the shrub. Some of the flowering shrubs may not bloom next year, but it does not improve the curb appeal to have a beautiful flower on an over grown shrub.

It may also be a good time to decide / remove if some of those overgrown shrubs may need replaced with cultiivars that do not grow as fast.

It depends on your current climate. As Embothrium said, spirea can be but to the within inches of the ground at this time of year. If you are in a place that gets larges amounts of snow, pruning now can be beneficial so as to prevent snow damage from snow loads. Like Mike Larkin said, if you have the time to prune now, and won't in a few months, it can be done now.