Check, check and check again. the level of the floors, ceilings, baths and shower trays. Never assume anything is level. Take into account anything that runs out when setting out the layout. I very rarely use batten now ( I measure and cut the bottom row from a level line and 'fine tune' with the small plastic wedges). But 'battening off' around the room will help the horizontal grout joints to marry up. Just keep checking with a level as you go and watch out for pipes and wire's when fixing batten, you can get tile tracker systems that don't require mechanical fixings but have personally never used them. Good luck in the new year and hopfully the tiler you work for can explain anything you may not be fully confident with and show you the ropes a little.