At times, fitting the composition to the aspect ratio of the camera makes good sense. OTOH, at other times, fitting the aspect ratio of the print to best present the ideal captured composition makes even more sense. And sometimes we really need to plan the composition of the print, and shoot a frame which is NOT best befitting the final print.
For example, a headshot might best fit a 8x10 ...but we hold a dSLR in our hands and it has a 2:3 aspect ratio. So we fit the head into the short dimension of the dSLR frame area, we ignore how much of the portrait sitter's dress bodice shows up in the long dimension of the frame area, then we print to 8"x10" (4:5) aspect ratio. But if the portrait sitter wants a 5"x7" print to give to granny for her mantlepiece, we print to a 5:7 aspect.
Another example, we see a shot while travelling, and take the photo. As shot, a lot of distracting elements are included in the dSLR frame with its 2:3 aspect ratio. If we mere cut off some of the distractions in both the foreground and some of the blown out area in the sky we have fewer distractions from the main subject and key elements of the background that we want, but it fits a 1:2 aspect ratio which fits no standard frame sizes or mattes. But the ideal composition fits within 1:2 aspect ratio, so we cut a custom matte to fit the 1:2 photo into a semi-custom frame using standard segment lengths (e.g. 30" horizontal x 14" vertical)
Simply printing to the same aspect ratio as the camera might be the wrong thing to do.
Simply printing to standard aspect ratio of commercially available print sizes might be the wrong thing to do.
Simply printing to 'best suit the composition' might be the right thing to do, even if the print size is oddball and it forces the use of a custom matte and semi-custom frame.