Lots of folks seem to worry about trees "bleeding" when they are pruned. Well, it's not blood, and plants cannot bleed to death. The main problem with any wound is that it exposes the wounded tissue to fungal attack, and once fungi begin to decompose wounded tissue, the fungal threads (mycelia) can spread away from the wound into other parts of the tree. I find that late winter is a good time to see the branching pattern of trees, which means I like it as a good time to prune. Experienced landscapers and nurserymen often say the best time to prune is "when the knife is sharp," in other words, any time of year. However, in winter, fungi are not usually producing infective spores, so there is less likelihood of fungal infection. Also, maybe it is a good idea to seal larger wounds, but only temporarily, not until the end of time. Plant wounds need to dry out to help discourage fungal infection.
Certain habits of pruning have become incorporated into bonsai culture, and you often will hear that such-and-such a type of plant should be pruned only at certain times. The truth is, over millions of years plants have evolved ways to cope with wounding (including the loss of limbs), no matter what time of year the wound occurs. Take pines, firs, and spruces, for instance. Christmas tree growers literally shear them to shape at various times of the year. The branches don't die back or fail to put out new growth; instead, new buds appear and put out needles, and the result is a fantastically shaped Christmas tree. We sometimes stress out too much about what to do and when to do it.
Don't know quite why I got on my high horse about this, but hope there's a grain of help here for someone.