I use a 50/50 blend, mostly because spring water is cheaper.
Does your tap water taste good? If so, just run it through a Brita filter. It gets to be a PITA to buy 10 gallons of water before every brew!
My tap water tastes pretty bad. I can usually find the natural spring water for $1/gal and the distilled for about that or $1.50/gal. I usually only buy 6-7 gal per batch, but yeah, it just adds onto the already not so cheap brewing expense.
Try to find a water machine in a supermarket or something. I buy 15 gallons of water in 3 5 gallon jugs for each 10 gallon batch and it's about $0.34 per gallon so really I am adding less than the cost of a yeast package to my brew and I can know more or less exactly what my water profile is because anything in there was added by me. Cheaper than a pH meter and a TDS meter for a quite a while anyway.