I have some taxus bushes that need trimming. Is now (July) too hot to do this? How about arborvitae?
Early to mid-spring is the recommended time for pruning yew shrubs, though you could do some light trimming only in the fall. Also, drastic pruning will greatly reduce its greenery, so any trimming you perform should be light unless you are willing to live with the loss of green for awhile. Yews can be cut back into older branches but new shoot emergence from old buds will take years and new growth will likely be thin and unattractive.
As for the arborvitae, they too should be pruned back in early spring prior to new growth. These plants, however, can take more drastic pruning and will regrow quickly.