@tunehead - Avoid them unless you have an issue. One more piece of gear to break.
DC should almost never happen in a home, and usually it's caused by things like LED power supplies and dimmer switches. Usually best to deal with the source of the problem and only if that can't be dealt with due to WAF and budget or being a tenant then a DC blocker is in order.
A better idea IMHO is to monitor your AC voltage near your rack and see how much it varies. If you see too much a voltage regulator may be a good idea.