I always meaure to find the centre point and dry lay tiles in every direction, that way i can tell if the walls are wonky and which position in relation to the centre mark to start. I always leave myself as big a cut as poss into the wall, never assume the walls are straight so never plan to run a whole tile along a wall. Cutting into the walls takes all the worry of being left with a small slither of tile, which as you said looks crap.
As Sir Ramic says get a set square, they are priceless, and draw onto the floor. Taking an hour or so setting out is time well spent if it means getting the floor to look right.
