Jessica - The VCE Shrub Pruning Calendar identifies box pruning can occur now in July, but you should avoid pruning box during periods of extended leaf wetness. Fall Nov-Dec is considered the best time to thin out your box.  Please see the following pruning calendar from VCE:

https://resources.ext.vt.edu/contentdetail?contentid=3183&contentname=A%20Guide%20to%20Successful%20Pruning:%20Shrub%20Pruning%20Calendar

Late season box shearing in Aug, Sept, and Oct should be avoided due to the potential for new growth to be damaged if we see an early frost. 

The following video from the Univ. of Maryland provides some really good pruning box guidelines:  

Improper shearing of box during periods of high temp can cause sun scald. Thinning out your box is to optimize light & air circulation in center of the plant.

Pruning is also a great time to closely inspect your box for disease and pests, particularly since Loudoun County had an extremely wet June which encourages fungal diseases. 

I hope this info helps! Thanks!