Just to clear up terminology - are you talking about one of the monsters that define and shape the edge, like a Little Wonder Bed Shaper, or just the spinning flat blade on a wheeled chassis? The Little Wonder type of thing is what you use once a season to get a really sharp edge on your beds. I've never used one, but I think something like that is overkill for a homeowner without an estate property. The Spinning flat bade thing is what you use on a regular basis for maintaining a bed edge.

My husband always turned our weed wacker upside down and used it as an edger, I don't know how often he edged. I could never master using the weed wacker but I sure knew how to mow the lawn and did!

I used one once to clean up my beds. But I took it a step further and after I cut the edge, I pounded in this fiberglass type edging flush with my grass. Today my bed/grass edges look perfect, I have no discernible mow edge and I haven't edged since.

A few years ago I did some research when I was considering a walk behind edger and I read that they work best when you have a solid surface to roll them on, such as a walkway or driveway. Even if it were self-propelled, you need a fairly level surface for it to roll on or it's going to be difficult to keep a straight edge.
I ended up with a hand-held Stihl edger and it worked well along the concrete edges but out in the yard it took some extra skill.

Our yard is nearly 1 1/2 acres and all the beds and outer perimeter are edged. We now pay for a partial yard care service and they edge the yard every 2 weeks during the summer months and the guy just uses a hand held weedeater. Not sure if it's a blade or string but it doesn't take him long to do it.

Once you've established the edge you want, you shouldn't see any increase in the size of the bed.
The only thing you're cutting when you edge are the blades of grass that have grown over your set boundary so you should always be cutting the same line each time.
The type grass you have will determine how often it needs cutting.

You may get more input on which tools are best if you post your question over at the Tool Shed or the Lawn Care forums.

Here is a link that might be useful: tool shed

I own a power edger and I used to use it to cut the edge of all the beds we maintain; which is quite a few. Then the crew and I decided it was just as easy to use a manual edging tool. Unless the edge is really overgrown and rocky soil, the manual tool is just as fast. On most properties, we cut the edge twice a year; once in the beginning of the season and once mid to late summer. I work in a similar climate to yours.
Using a manual edging tool, you push it down into the sod at the point you want to create an edge and the key is flicking the tool towards the bed so the sod comes up and out. then it is just a matter of cleaning up the sod pieces and its done.
The power edger is somewhere in the back of the barn...haven't used it for a couple of years now despite the miles of edging we do.