It won't sour if the temp stays above 135-140F. That's also because there won't be a lot of lacto left after bringing the grist up to 168F. You'd have to inoculate it with raw grain and drop the temp to about 130F or lower for it to start souring. This is why overnight mashing works. I've done it a number of times. Letting the sparge sit overnight isn't much different. My recommendation is to make sure you get the temperature of the grist up to 168F, so it will stop conversion. This means you need to infuse the grist with 180-185F water to get the grain bed to around 168-170F. If you infuse it with 168F water it won't really change the temp of the grain enough to stop conversion, and your beer might attenuate more than you want it to.