Many years ago I never bothered, but there was an occasion on a site that the kitchen fitter mixed up the work tops in ajoining houses being fitted at the same time. He was a really likeable chap and I'd just tiled both kitchens with metros when the mistake was noticed. Both worktops had to come out, which meant the bottom row of tiles also had to come off. If I had of tiled with a spacer below then there would have been enough wriggle room for the wotktops to come out without much disruption. As I felt sorry for the fitter I did the repairs for free to help him out and since that day I've always tiled off the worktop with a spacer below, even with metros. It costs us nothing and apart from the obvious implications of exspansion it might just help some chap out some day.