I use a 4x6 block techic because I carry them anyway plus in 4 low its really easy to put it on a block.
This method is the safest and fastest in my opinion because its the fastest way to get the blown tire in the air and there is no jack to fall or fail. I think it takes me longer to lower the spare than to change a tire. My 5th is a strait forward leaf spring design. I just have to make sure I loosen the lugs before I put it up on the block.
I am of the opinion that the least amount of time on the side of a busy highway is safer and beter even if it means extra stress on the trailer axles and tires.