like most things there are no real rules - but there are probably 4 ratios that are common
4:3 - the native ratio of a 4/3 camera - and standard definition TV
3:2 - the classic 35mm film format ratio
1:1 - Square format - used by old medium format film cameras
16:9 - the widescreen TV format.
probably more important than the ratio is to make sure the elements in your picture fit in the right place. A good rule of thumb is the rule of thirds
Rule of thirds - Wikipedia, the free encyclopedia
Its an excellent starting point in deciding how to crop
Don't forget 6x7! Though I usually shot 6x9 since it is the same ratio as 35mm.1:1 - Square format - used by old medium format film cameras
If you plan to get a quick print from a self-service machine in a supermarket (where a choice of aspect ratio is generally not given), a 2:3 image will suffer from practically no cropping.
You have to be careful with framing by zooming, ,since it changes perspective which changes the message of the picture. Same with cropping, of course.The biggest reason I crop is probably just to get rid of negative or wasted space. While a zoom lens can do the same, sometimes I don't know what will look better until I see it on a monitor. I'd rather have too much than not enough. Once you take too much, you can't get it back(kinda like a haircut, LOL).