There are times (rare) when I buy fabric from the same line to use together, but that is usually more for what I call a planned quilt instead of my usually more scrappy project.
I don't think I can give you an answer, but I think that's one of the great things about quilting is we all get to find our own answer to work for us. Each of us has an internal "makes me happy" (or cringe... try for happy!). Each project has it's own set of rules or constraints. We can emphasize fabrics by putting them together, or keeping them apart.
When I'm looking at my inspiration fabric for a project I look at the amount I have and what I think I need. While I try to keep scrappy, I also try for some balance and consistency. So say I have a bit of a particular red, enough for maybe 5 blocks but that I will need 30 blocks total. I'd want to have at least 6 red, or if I had a lot of little bits, maybe I'd be willing to have 5 of the one I loved, and maybe 3 each of a few others, and then a few more entirely random.
I'm dealing a bit with this right now in my Bonnie Hunter project. Is a mystery and the final has been revealed, but I'm still in progress and it's still a mystery to me!