Masking will always make the image look better and the presentation look neater and more professional. Even with projectors with the better black levels (like my JVC projector) masking still pays dividends.
But the "need" for masking tends to be subjective. That is, masking will just about always look better, but some people can live without it much more easily than others. I personally can't live without masking on my own system, so it's up to you how important it is.
As for the expense, if you don't want to buy a commercial masking system, DIY options run from, like, $50 (or less) for some foam board covered in velvet that you can place on and off the screen manually, through to options using curtains and curtain tracks.
Depending on how your screen is positioned you should be able to run a curtain track along the top of your screen. Simply setting up some dark curtains to pull along for side masking would be super cheap. (Some people sew in wooden dowels or the like to the inner edges of the curtain, to keep it straight for a crisp masking line).
You can get fancier by buying an automated curtain track with remote control. These can actually be found pretty cheap - a couple hundred bucks or so.
Personally for my side masking I wanted it remote controlled, with programmable pre-set stopping points for various image widths, and a crisp masking line. So I spent a bit more - about $1,500 because mine set up was really large and fairly complex, with 10 moving panels. But it works superbly and still costs less than many commercial systems.
If you are really handy you can build your own motorized side masking. Look in the DIY CIH forum for examples. But it's tricky, and time consuming...a labor of love thing that isn't everyone's cup of tea. I preferred to go with a professional, reliable, quick-to-set-up system myself. But then, I'm not nearly as handy as many people on this forum.