If you know you are going to be cropping for a special size, fill the frame, when you compose the shot, but remember to leave enough negative space for your final, planned output.
For the web, most anything goes. I typically use a 2x3 or 3x2 width to height ratio, but I do mix it up for variety. Panoramas tend to be wider and have strange ratios, depending on the number of frames that I capture in a panorama. A Scott Kelby trick that I mimic is to use a 16x9 crop for a cinematic look.
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