Ideally, your game should be able to fill any (reasonable) aspect ratio, whether it's 16:9, 16:10, 4:3, 5:4... This means accepting that sometimes, the screen won't contain a whole number of tiles and some tiles will be displayed partially. Depending on the gameplay, this might not be a problem at all (anything with a scrolling screen, e.g. platformers, RPGs), or it could be a huge problem (puzzle games relying on the grid). In the latter case, you can solve the problem by having a fixed size/ratio for the gameplay area, and a flexible frame around it that has other info and menus.