Never use them I use my cross line poll laser level witch will send line round room .......work from floor up and those coloured wedges to pack up from floor if necessary
Battens for me too.....I used to use the 8mm ply cut ones till I reduced them all to be too small to be useful...I then got others cut at a wood yard. And found to my annoyance that they all bowed slightly..guess the saw was knacked..tapered vertical groutlines aint good !
I still use battens and like tedley make the bottom row the last. But I now use MDF architrave instead of ply. You can get quite a few for a similar price to a sheet of ply. But I have found them always to be absolutely "perfectly true" as they are manufactured and dont need cutting into strips where bows can be introduced or they dont have a natural grain that will make a softwood batten deform even if it is PAR. They are good and sturdy too.