Sorry... he was supposedly quoting the service manual... but really wasn't. I happened to agree with Safety Man's advice, but that's just my opinion.

Didn't mean to offend...

 

Use all the new o-rings you want man, knock yourself out. A great many years of direct mechanical experience has taught me that it isn't always necessary and there's really no need to get all **** about it.
While you are absolutely correct that an o-ring when seated is slightly distorted, it is also true that buna o-rings have a memory. When you remove the plug it does not remain distorted but returns to it's 'as new' shape. Only if the o-ring has been left in place for an extended period and/or over heated will it become hard and brittle requiring replacement because it is forever flattened.
I replace o-rings when they are in a location that requires quite a large tear-down to get to them (stacker valves, etc.) - only makes sense.
A Harley oil plug isn't one of those places.
Also I have service manuals for nearly everything I have ever worked on. One thing they all have in common is a recommendation to replace nearly every 'soft' part as well as many 'hard' parts.
One need not be an engineer to realize that replacing everything the book calls for as being an 'absolute'. Direct experience and common sense is what separates the 'absolute' from the 'not always necessary'.
But there are those that do everything 'by the book' regardless, and that's OK. You fall into that category and there's nothing wrong with that.
You also display an arrogance when it comes down to following the book - like just because you do, you are better than everyone else. Not true, but go on fooling yourself. With that attitude you'll never really learn mechanics, you'll just go through the motions the book calls for.