iamduvern,
Like many things the answer is largely dependent upon weather conditions and nectar availability in your local area. If you do a shook swarm too late in the year the colony will not have time to build comb, raise brood and gather in sufficient stores to last them the Winter. If you feed them you will help the process but ultimately you need to consider conditions in your local area.
One beek of my aquaintance plans to place brood boxes containing fresh foundation on top of some of his hives this August and to feed them to encourage them to draw fresh comb - not quite the same as doing a shook swarm I know, but the plan reflects his confidence that the bees will be able to draw a box of foundation in August and gather in their winter stores.
Those I know who practice shook swarms as part of their programme of pest and disease management seem to use it once the Spring nectar flow ends and feed their colonies to help them draw out fresh comb.
I too would be grateful for comments from long standing beekeepers as to what they consider is their favoured time of the year to carry out a shook swarm.
PS - as milkermel notes, it would be a good idea to quantify the extent of your varroa/nosema problem before rushing to treat the colony.