I change my own tires and I normally change them when they have each reached their wear limits.

 

+1 on that! I dont throw money away,Damn tires cost to much now!

 

Can't always find the flaw in the new tube either. Fact is a new tube is no safer then using a well checked used tube.And one should always check out a new tube as well before install. But if you feel safer by all means change them.
Ways to check tube status is to air it up slightly larger then tire. Checking for cracks, thin spots and rub wear etc. Dunking it underwater will find any leaks. Checking it deflated will not find much.

 

I think you should blow it up until ½ lb of pressure before it pops, this may take several tries to get it right, but hey, you're worth it right!?!

LOL, just kidding, I agree, check the tubes as TheMoparMeister says, old or new before mounting.

 

It has always been a standing recomendation that if you change one tire, to change the other at the same time. How many out there actually do that? Also, do you really need to put in a new tube every time you change tires.

WHY ???