There are several reasons for backfiring (techically it's an afterfire)
2 most prevalent are lean carb adjustment, and exhaust leaks.

If it backfires on shutdown or idling, and I mean kaboom not just a puff.  then most likely it's the pilot adjustment.  Look on the right side of the bike, right side of the carb, there should be a brass plug if you have an unadulterated bike, otherwise if the plug has been removed, you'll see a screw.  See lancers straight and simple tuning guide for how to adjust.  If it does a puff at shut down the adjustment is either a shade rich or just right.

If it backfires on shift, every shift, check for exhaust leaks first.  Then you can try increasing your jets or the white spacer mod.

I have finally tuned out the backfire on shutdown and have achieved a smooth idle.  However, I still experience puffs/backfires (not too bad, but enough that I want to fix it) during shifting.  I've done the white spacer mod (it's about 1/4 of what it was) and have changed my main jet to a 150.

I think our puffs/backfires during shifting may be due to the pilot jet?  I'm gonna do some more tuning with the idle mixture screw, but I may wind up having to go up to the 55 pilot jet.

Are you going to have a dealership do this work for you?  If so, they may not re-jet your carb and/or adjust the idle mixture.  The reason our carb (I have an '05 too) is so lean is because that's the way the EPA demands it.  It may be against the law to mess with the jets/idle mixture.  Not sure....

does he mean a poof kind of backfire or a KERPOW kind?
poof I'd accept, KERPOW I would not.

In my experience (however limited) backfire on shifting was caused by exhaust leak.  And it did Kerpow on shutdown.  Tightening the header bolts fixed both.

I also have a 2005.  Jerked out the white spacer and replace with a 3/33sec size washer........pulled the plug and adjusted it out 2 full turns.
Also found a small leak between the header pipe and muffler.  This was well over two years ago and no problems.  Header pipe has not turned gold or blue.

RIDE SAFE!!!!!

Fixed the leak when found............Get a little puff after a long ride at 50+ miles an hour, but expect that..  Loud bangs?......No.

I have been thinking about an after market muffler, but I don't want one I would have to tinker with the carb all the time.  Seems like some on here differ as to the rejetting.  I'm aways open to suggestions.

I put the 'mojo' on the bolt...........If that did not solve the leak I would have ripped it all off and replaced the header and muffler gasket and probably added some gasket sealer from Autozone.

If you are going to use a lubricant (ie antisieze) then use the lowest end of the torque specs, other wise you may strip the head threads. Unless noted in the shop manual specs and I've not see it, the specs are for clean dry threads. Max