I just got a DuBro balancer.

I've searched online and it seems split on whether to sand or add tape to balance a prop.
Same with the hub. Sanding or some add glue.

Is this a personal preference I guess?

I've seen videos where tape is added to the top and some videos added tape underneath. Where would be the best?

I add tape.  No particular reason other than it seems easier.  

I sand.  Tape and glue might drop off.

If you use genuine DJI props, you will be removing/adding very little material.  Aftermarket clones are usually badly unbalanced in comparison.

If you sand, you can sand off the imperfections, rough edges, the dji logo etc which will make the props quieter and more efficient.

I think the only place that tape is preferable is if you need to balance the hub, the aerodynamics there are not so important since it moves at relatively low speed but strength is very important.