If I see a DUI checkpoint or roadblock ahead of me and I turn around to avoid it, will the police assume I'm avoiding them and come after me?

Yes, they might come after you.  In Fairfax County, many checkpoints are set up so that they have advance notice, and there will be some natural intersection where you can smoothly turn off and go another way.  Cops know that drunks don't usually read the signs very well and don't think fast, so the folks who turn may just be "let go."  

If you turn into a driveway and turn around, or make a U-Turn at a Fairfax County DUI Roadblock, then you will probably be more suspicious.