Reaming inside and outside of neck might be advantage if you have slightly sub-sized custom neck in your barrel that requires that - with the appropriate expander ball and sizing die (or bushing die) that goes with that. I have loaded since mid 1970's - I played with outside neck trimming for a factory 338 Win Mag rifle - I could not show myself any difference in 5 shot groups at 100 yards, so I decided that was "busy-work" - waste of my time and effort, if I could not produce results on target. Outside reaming may have merit to "even up" the neck thickness, but that thickness flaw is going to likely run down entire case body. I have not reamed at all, since early 2000's. If you are concerned about "bullet neck tension" - measure your case mouth wall thicknesses - sort your brass by that. If you want something to fuss about, look into loaded cartridge concentricity and how to measure that - I most definitely satisfied myself that 0.003" runout (or less) gives me better groups (with my store-bought rifles with factory production chambers and barrels) than 0.004" run out (or greater).
If you have reamed your brass so necks are now too thin for your chamber neck, expander ball and sizing die, then those brass are "ruined" - load them up as practice rounds, until the primer pockets get loose. Will be a LOT of difference among what is useful to do if you are 0.01% top bench rest or F-Class shooter with highly customized barrels and chambers, versus "normal" person using store bought production stuff.
In "Modern Reloading Second Edition" by Richard Lee - he suggests that you want the bullet to be about 0.002" LARGER than your sized diameter of your case neck - that is the "bullet tension" that you can achieve - as per him, making the neck to be smaller, simply stretches the brass in the case mouth when you seat the bullet - you will have gone beyond it's elastic capability, and will actually get SAME or LESS neck tension by making your case neck ID smaller - not MORE. In the end, it will be the holes that you produce on target that tell you whether you have improved things or not - holes on target do not lie.