I don't think there is anything wrong with always keeping your head in it, thinking if there is something missing or something you should be doing. If you aren't sure if you ran a checklist or a flow right, run it again. That's being diligent and I don't see how it is an indicator of being unprepared.
Distractions come up all the time and can interrupt the normal flow of getting an aircraft ready or ensuring safe operations... fueler, ramp, gate agent, purser, maintenance, weather, load, dispatch...
I often run a quick second check in my head after I think I got it all or am all set up and ready to go. For whatever reason, **** gets missed sometimes, better catch it before you push the levers up and get the horn or worse...