As far as I gather, the only difference between sleep mode and turning it off is that in sleep mode, the Kindle will still check the wireless connection to see if you have any subscription material to download (if the wireless is turned on, naturally). I normally leave it in sleep mode.
I'm not sure that the Kindle is actually "off" when the power switch in the off position considering how long it takes to get back to operating mode when it's reset. Being a long-time computer geek (got my first computer in 1981), I realize that Linux does a really good job of "garbage collection" without having to restart, but I just feel better doing a reset once a month, just as with my Mac and Windows machines.