What aspect ratio you should use when cropping depends on how you will use the final image.
- If images will be viewed only on screen or projected, use the screen or projector aspect ratio. Newer screens are usually 16:9. However, 8:5, 4:3, and 5:4 are also possible.
- If images will be printed, use the print aspect ratio. Aspect ratios include 3:2, 7:5, 5:4, 14:11, 6:5. Aspect ratios that don't match standard print sizes may be used for double-page spreads or if you don't mind trimming.
Whether a client is bothered or annoyed depends on the particular individual.
- Aspect ratios that differ greatly across images could bother someone who is very particular.
- Differently sized images with the same aspect ratio are unlikely to be noticed because they are all resized when viewed on screen.
Personally, I leave the aspect ratio the same as what the camera produced to avoid double cropping when printing. If I changed the aspect ratio, but decide to return to the original aspect ratio, I'd have to crop again.