How high quality you bourbon should be really depends on what kind of mixed drink you're making. If you're doing something that's fairly "whiskey forward", like a mint julep or old fashioned, then some higher quality stuff is in order. If you're just making bourbon and cokes then Evan is already the Cadillac of mixers. Old Grandad bonded is a good choice as well, and WT 101 also, but that's good on it's own.