Pruning larch and spruce

I have 20 acres of 25 year old  larch and spruce. I would like to prune about 10 percent of them to improve the quality of the timber 50 years  down the road. I have heard that pruning green branches can lead to infection unless done at the right time of year.

     When can I  prune safely?

A second question is this: can I fall about ten percent of the weakest trees without getting them down on the ground  or can I just leave them stuck up in the canopy.  They are light and get stuck in the canopy and are not dangerous enough  to be considered widowmakers.