We ferment in a rock/wood cellar, There are abundant wild yeasts there, the only problems i have with cross-contamination is when I open the fermentors for an extended period in the cellar. Typically if I have something open in the cellar for that long, it has some problem anyway, so it ends up getting inoculated with the bugs and I will simply add it to my wild brews or start another one.
If I want to check gravity on a saccharomyces only brew, I will remove it from the cellar, it's typically ready for bottling when this needs done anyway. I use all the same stuff to handle the wilds as I do the more conventional brews.
Newer, better, more streamlined sig as per the forum police.