As I was a contributor to Bullpups post I will give my 2 cents worth and then leave it at that. Also have been an amateur competitive bodybuilder in New Zealand for the last 4 years.
Everyone seems to have their panties in a twist about this one.
Measurements are used to gauge progress, correct? So what is the big deal with taking both relaxed and flexed arm measurements? A relaxed arm measurement may not be as cool as a flexed one but it can be an indicator of progress none the less.
Something else i've noticed is that a large number of people couldn't do a decent bicep flex to save themselves. Poor shoulder flexibility, tight pecs, hunched over posture...not the most accurate way to get a measurement if you're all twisted up.
Whichever way you want to take it is fine. As long as you're measuring improvement and it's helping you plan your training accordingly.
If you have a response to this could you PM me please. Thanks