i don't use sleep mode at all, my friend used to let it drop into sleep mode. not sure what he does now. some hardware combos _do not_ like it. it's sort of a preference vs compatibility thing.
now; the antec 550 has a 9A input
9x120=1080w input
here, the cost of power is something like, 5.71 cent/kwh, which factors out to just shy of 1.50$/day or, 45.20$/month (30days)
keeping in mind this is _full_ 9A draw, which isn't always going to happen, but it's safe to use that as the baseline without hooking an inline amp/watt meter between your pc and the wall
sleep mode will certainly cut the cost down by at least 1/3, to 1/2, depending on how often it's in sleep mode and how much is 'sleeping'
in the end, it comes down to choice ^_^