Yes, you want to have headroom, that way the masterer has headroom to play with. You want to be at -18 dbRMS which is 0 dbVU in the analogue world. Get yourself a plugin like Satson or klanghelm, or figure out how to set this up in your DAW, the needle should tell you when you are hitting 0dbVU which is theoretically what you want to hit. A trick, some people do, is load up a limiter plugin in the masterbus, something like Elephant, give it a gain boost like +7 from the getgo (that way its loud which causes you to mix at lowers volume on the tracks), then remove this plugin at the end and stage it at 0dbVU when being sent to be mastered, some people like this some are against it because boosting can add color. Also, you want your source tracks when you record to be at -18 RMS thats the average of course, usually -21 RMS to -18 RMS. Hopefully this makes sense.