Gosh! I'm torn on this one as a CFI. Personally I would suggest going to Foreflight once you hit the Cross Country phase, but really, as a PPL knowing how to use a paper chart (although quickly becoming less relevant with all of the EFBs) is really important. I find that my students (70% in their early 20's...since I work for a community college) have a tendency to overuse their iPads and not have a better understanding of what to look for outside or how to correlate what you see outside to what you see inside.
As a CFII, however, its sometimes scary how useful the tool is to me as an instructor. It allows me to keep a perfect picture of where we are, where the student needs to go, and any indicate errors before they get out of hand. Pro is a VERY powerful tool for a IFR certified pilot.
Personally, I would say no Pro just yet. I appreciate Ron's argument for Airport diagrams, but Standard has those diagrams too and as long as you can see outside the window and keep a good sense of direction, a picture of the airport on your knee is a sufficient.
(BTW...Foreflight does automatically pop the Taxi Diagram when you start moving on an airport)
And since you are in Oklahoma, there isn't too much in terms of terrain to worry about. I can vouch for that being in Florida, the only thing that really concerns me is the 2000AGL towers scattered about. So Pro isnt too necessary for that.
Once starting IFR...PRO if used correctly can be very powerful!