You really need to shoot both. Some people can't shoot the bottom barrel first or even as good as the top at all. Personally I agree with Irv and that's the way I shoot but after trying top first for a while.
There is a lot of good advice here but you have to try each and see what works for you personally.
I took a class from a well known instructor and his advice was "I give you the tools to work with but its up to you to try each one and find which works for you".
Like the others have said this is not carved in stone. I shoot an MX-3 that is designed to be Top first. I owned a Citori Trap for 14 years and also shot it Top first because my scores were better that way. I never noticed any difference in recoil and both barrels were pretty close in POI.