Do FAA regulations keep aircraft away from 18,000 feet MSL to avoid conflict with FL190?
It's not the altitude that is adjusted, but the flight level. Aircraft are allowed to fly at 17,500 feet MSL. Per the rules provided in other answers, this would be VFR flight to the north or east. an IFR aircraft could be flying at FL180, and this would provide 500 feet of separation.

However, 14 CFR 91.121 determines the lowest usable flight level. If the altimeter setting is 29.92 or higher, FL180 provides at least 500 feet separation from the next VFR altitude, and at least 1000 feet from the next IFR altitude. However, if the altimeter setting is lower, then the lowest usable flight level is higher than FL180.

Current altimeter setting   Lowest usable flight level
29.92 (or higher)           180
29.91 through 29.42         185
29.41 through 28.92         190
28.91 through 28.42         195
28.41 through 27.92         200
27.91 through 27.42         205
27.41 through 26.92         210