Yup its doesnt matter ..
but something that most of us noticed since TB started is :
- if you stay at spec , we will take 8 mins as example , so (30-8)=22 min normally rating should be based on the 22 mins you played , well its doesnt work like that ( dont know if the bug got fix or no , since we didnt talk about it since few days) the 8 mins you were at spec counts as 0 XP in your rating session .
So you got XP only during the 22 mins you played ( we take as example 400 xp) but the 8 mins stats counts too so 400 (22mins) + 0 ( 8 mins at spec so 0 xp) = 400/30 * instead of 400/22
To resume even if you are at spec , it counts for ur rating , as i said dont know if its fixed or no yet but just in case use another name while you are spec .

* it doesn't work like that but just to make it understandable .