Is this technique best use in conjunction with a depth finder? I've never used it before, but I would guess you need to have some idea what depth the fish are holding at to properly use the drop-shot.

Is this the correct assumption; or am I missing something?

More often than not, I throw it in open water and work it back to the boat. Most of the places I fish average twelve feet in depth...

Direct answer:  Yes and no.

If you know the body of water well, then having electronics isn't nearly as big of a help as they are on unfamiliar waters.  If you're vertically dropping the lure, having electronics can really help you pinpoint fish or structure to drop on.  If you're covering a broader area, using the cast and retrieve method like Matt mentioned, the electronics aren't all that important.

If you've got them, use them to your advantage.  If you don't understand your gear though, it doesn't matter what you've got in the boat.  You need to be able to distinguish fish from rocks, or fish from weeds, etc etc.