His atty Orings come in two sizes. 1mm x 10mm and 1.5mm x 10mm. And yes the different sizes make a difference.
I have found that not using his modded attys I can use the 1mm x 10mm ones, but for the non REO attys, I have to use the 1.5 x 10mm attys.
The REO attys will not make a connection inside the 510 without the 1mm x 10mm oring.
The danger of not using an o-ring is that if you ramp the atty down too hard without it, that you will ruin the internal gasket in the 510, not that it will not make a connection. If you do that - it's a trip to Rob.
There are 2 places you can use an o-ring on say a RM2. One is under it, in the drip well - and that allows you to position the airhole to your liking (i.e not in your face!)
The other is between the cap and base of the atty - and that can be useful if you are trying to make the airhole align with the coil inside the atty.
Don't forget that a lot of atties have an adjustable centre pin, so making a connection is really not a matter of using an o-ring or not, since that would obviously increase the distance needed to make an electrical contact.
T