I prefer the IBNS standard until it gets to the UNC category. Whereas there can be ranges for all other grades, many think that "it is eaither UNC or it is not". I'd be Okay with that BUT - most everyone else is not. It is a very scarce dealer/seller who grades an UNC to the "it is either UNC or it is not" level. Most will call notes UNC with a small corner bend, bumped corners, counting marks, etc. It is also accepted by many collectors as well. So I have also learned ot accept the fact that there is a range to be had with UNC notes as well.
As for the TPG grades, I have said it before and I'll say it again - There needs to be a "third Party" (IBNS?) standard that TPG's should be held to for their grading. Having different standards for each company, even if they are close, is not acceptable. In my past experience, I have had to "grade" things that I would inspect - Long ago, the field I worked in adopted grading/inspection standards to ensure that there was quality behind them, and that one acceptable item would pass as acceptable anywhere else in the world, without question. Inspectors were also certified to that standard. A smart company would be able to set this up, make an acceptable stnadard. Then, enforcement would be up to the public, if they all wanted to have 'certified grades to a set standard by certified inspectors, no matter the company. If it were me, I'd also get away from the 70 point scale. I understand its been around for a while, but why 70 instead of 100??? It bothers me, I tell ya! 70 is a "C" grade in my opinion - but that's just me.
My rant done, I guess I use a combination of generally accepted standards, defaulting to the IBNS standard in most cases, especially as I collect world notes, and not just US notes.