Hi Aeronwen! You are right when you assume that the corner wedges are to tighten the canvas across its stretchers. When I paint on canvasses, I tap in the corner wedges when the painting is finished and dry.
However, be careful when you do tap them in, as it is possible to overtighten and burst the edges of the canvas. Also, note that there is a right and wrong way to fit them into the corners; if you look into the grooves at the corners of the stretchers, you'll see that there is a slope on one side of the slit; so position the wedges so that the angled edge of the wedge "slides" down the sloped angle inside the slit in each of the corners.
I hope the above helps, but come back to me if you need more help.
Very best wishes,
PA & RSC Wigan / Warrington / St Helens