I have all my prescriptions at PetMeds and then before I order a refill I do a google search to see if the meds are available someplace else for less. If they are, I order at PetMeds and then send them an email asking them to meet the other price. I send them a link and they have ALWAYS honored the cheaper price by taking off the difference on my invoice before sending in the credit card number for payment.
In this way I don't have to deal with having prescriptions on file at many different places.
I use SmartPak for most of the heartworm meds and the big guy's Deramaxx, and KV Vet for anything else. In terms of price, I've found that most of the big places are within a couple dollars of each other once you factor in shipping, and since I'm in the MidAtlantic, SmartPak orders get here VERY quickly, so they get most of my business.