Personally I like to mix solids with batiks, I think it makes the batiks stand out a bit more...but maybe that's not what you want in your bargello! I've finished one and am working on a second bargello and I mixed batiks, solids and prints in both of them. But I also tend to like really scrappy looking quilts.
This isn't a very helpful post, is it? LOL I think there's no wrong way to do it - if you find the right batiks to give you the colorway you're looking for, I think you're all set!