Keep the pressures at the recommended figure, over-inflation will 'bulge' the tyre tread, reduce your grip on the road and cause the tyre tread to wear more in the centre than on the outside edges, so the money you save on fuel will probably be partly cancelled out by buying tyres more frequently.
The tyre pressure will naturally go up and down a bit according to the daily temperature anyway, and it will be lower when weather is cold and higher in hot weather due to the air expanding and contracting, but we really can't do anything about this except to check pressure fairly regularly.