Propane tank recertification

My propane tanks (30 lbs.) will need to be recertified in a couple of months. Does it make a difference what State they are recertified in? I'm from Minnesota but will be in Alabama when my date comes up.

Also, do the tanks need to be empty or can they be somewhere between full and empty?

I called our local propane dealers, one said empty one said doesn't make a difference. One wants $5.00 the other $10.00, what's the going price.

My thinking is if I have them filled now (home) that I will be shorting myself by about 6 months before the next recertification.

Just wondering what others are doing.

Thanks.

Paul...