Been there - have hit both a gas line and 480 volt electric line that had been cleared. Now, if I'm unsure, I tell the client I'm not doing work until the location is hand dug for five feet or have used hydro-vac to check. Too often the line finder is off by several feet horizontally and safety requires an abundance of caution.
Oh, BTW, not all companies are members of the 811 system (though they are supposed to be in NM) so checking as outlined in your post is prudent.