if you do it that way any crap that has collected in the caliper will be reintroduced to the system and may cause problems. on some vehicle braking systems this is more of a problem than others.
if you have ever takin brake fluid out throught a nipple then you must have seen all that black crap that comes out.
generally i just push the piston back unless its time fer a fluid change, which due to the hydro-scopic properties of brake fluid you should do every two years