With tomatoes (indeterminate varieties, anyway), one rule seems to be true:  Prune early, prune often.  The plants waste no time sending out suckers in the joint between the main stem and existing branches.  We discovered last year that if too many of these grow into full-sized stems, the plants become bulky and more difficult to tie to their supporting cages.  Also, the more branches there are, the less energy is left to set and develop fruit.  Further, bushier plants give tomato horn worms (and other pests) better places to hide from view; when they become visible—by eating the leaves—it is too late.

Therefore, we started pruning today and will continue on a daily basis, as needed.  If we keep on top of it, it will not be pruning per se; it will be more like preening.  When the suckers first appear, they are very small and need only be pinched off between thumb and index finger.  It can be as enjoyable as deadheading the flowers or grooming the cats (and if tomato plants could purr, I’m sure they would).