2 or 3 Wire Power Drops?

No.

As Rob said, No.  Just wire the common "jumper" to the outside rails on the same track section as the 2 wire power drop.

Tony

I wire one common to the right rail then at your next drop,  wire your next common to the left side, your metal wheels are also connecting the outside rails. 

Jack

 I think the answer is a little of both...We have a large DCS layout and drop a wire from each outside rail, join them together underneath  and run them along with the hot back to the terminal block/. (paired wiring)

This makes for a solid common . Just using jumpers may eventually lead to a lost common especially around switches,

This helps most  engines with traction tire and poorly designed 2/3 rail engines ability to pick up the common.

Our 8 PWZWs are also phased  together via the "U" connections.

TMCC & DCS runs great . Every now & then we may remove a feeder for signalling relays. etc.

I have a 16x35 foot layout with all Atlas track and only used one rail as a common and the center rail as the power.  I use the other rail as an insulated rail for activating the Atlas O's 21st century signal system.   I have bus wires run for all three common, power, & insulated rails.  On the common and power, I have drops every six connectors and on both sides of all switches.  I am using TMCC only and have no problems power lags with running as many as 4 locomotives on the mainline.  

When I designed my layout, I divided track into signal blocks even though I didn't have $ignal$ at that time.  But I put in insulated pins and dropped three wires below the plywood and joined the outside rails (actually insulated all three rails which then ran back to my panel, because I had no way of knowing if there might be a unique future need for insulated outside and center rails (yeah, overthinking/design, happens when a retired EE)).  When I had saved my pennies, I bought the Custom Signals system and modded MTH two head signals placed at crossover interlocking.  It was very easy then to separate the insulated rail for activating the signals.  Sure saved the hassle of trying to tear up rails etc. 

Granted we may not know on our "perfect dream layout" what we will add later and might have to do MOW work to add/extend blocks to make signals and accessories work.  But in a perfect world.....

Thanks for the input.  I thought I recently read that having an electrically isolated rail section (insulated) will, over time, causing some issue - maybe pitting of the rails?  Peter