Most SLR camera viewfinders don’t actually give you 100% coverage of the image area. Cut-off between 2% and 5% is common. The exact figure will be found in your camera handbook. In practice this means if you position something right at the edge of the frame when you get your photos back you may have unexpected space at the edges of the negative or transparency. Coincidentally, commercial printing equipment masks the edge of the negative during printing and may actually cancel out the aforementioned cut-off. It may now seem a bit pointless telling you this but you should be aware of it, particularly if you are going to print from your own negatives or you are shooting transparency (slide) film.
There’s more. If you are having your prints processed commercially then you should realise that the proportions of a 35mm negative (2:3) don't always match those of some popular print sizes. If you want your prints to match what you saw through the viewfinder you will need to ask for "full frame" prints.
As a wee example: A 35mm negative printed to show the full frame onto 10x8 inch paper will produce an image closer to 10x6 inches.