Planning on using T-58 for a wheat ale that has 20% rye malt in the LME, about 1 oz of lavender and some Briess Dark HME. Using Mr. Beer, it's a 2 gallon batch of an Americanized dunkelweiss...
You really don't want to filter AFTER carbonation has taken effect. Any form of filtering or aeration could flatten the beer and/or lead to infection, and you don't want that in the bottle.
Fairfax County water is pretty drinkable, though it does have some additives.
I have a Pur filter on my kitchen tap. I'm using Mr. Beer for my brewing currently, and all my water is filtered.