It is always best to compose your photos so they don’t need much cropping but there is also something to be said for leaving just a wee bit of extra space around your composition to allow for just a touch of cropping.
I’ve had photos where the perspective was just a bit off (if you can’t get directly in the center of an object, the perspective will be off). With Photoshop I can correct that. Unless you have that program, you probably won’t be able to but if you get right in tight on the subject, you may lose part of it from looking at a different part of the subject or it may be in so close that a slight perspective issue will look more pronounced.
If you step back just a bit and leave just a bit of room to crop, just in case you need it, that will sometimes mean the difference between a good photo and a VERY good one.
As with other tips, this one is meant to be broken and on a very regular basis. There are times you will know you don’t need to crop, that cropping would be silly for this one (a sunset or sunrise, a specific part of an object, even someone’s face) and then you should feel free to take your photo as close as you like.
The main reason you don’t want to do any more cropping than is absolutely necessary is that each time you move in closer to the subject in your development program, the less resolution your photo will have. For most photos, especially those that will never get larger than 4”x6” that’s fine. If, however, you’d like to blow this one up the 8”x10” or 8” x 12” then clarity really is very important.
This becomes even more important if your camera is a few years old and it has 3 or 4 megapixels (or less). The fewer megapixels, the more difficult it is to enlarge a photo clearly and then, when you add in cropping it, the results can be less clear than you’d like.
In a nutshell, crop when it will improve your photo but do your best to set up the photo so it will have the look you want while you’re in the field.