Mines done underneath.
Any excuse for a night on the tiles:)

Tile the whole floor while the room's empty. Trust me. Not only does it look better it levels the floor better for fitting the kitchen. In the last house I lived I did the whole house top to bottom including the kitchen and tiled the whole floor whilst there was nowt in there, and in addition to the above points it also makes tiling the floor easier because you've less cuts.

And if you haven't already got one, get a tile cutter, one of the electric type wheel ones they're cheap as chips and worth every penny.

Thing is its like wallpapering behind the bedroom wardrobes.....nobodys ever gonna see it. My tip - if yer gonna do the whole floor dont fanny about too much with the one's which are gonna be hidden. That's if you know at this point of course.

Remember that if you tile after the kitchen goes in you'll probably need to buy less tiles (depending on pack sizes). Just leave the kickboards off and tile up to the feet of the units and than cut the kickboards to suit. More fiddly but less tiles to purchase!

Valid point but practicality will be a clart on if your kitchen floor isn't already level.

Plus if you tile it right through now then the tiles are there for the duration if you want to change your kitchen units again in a few years time and the units are differently sized.

The other advantage if you lay the tiles right through with a proper grout you're basically waterproofing the floor too if you get any leaks, water run off etc down behind the units so all in all worth the extra couple of quid in tiles to do it properly while you have the chance rather than a botch job in my opinion.

Just use sawdust.
It will give it a much more authentic feeling and if the dog shits or spews in the kitchen you can just shovel it up and put some more down.