If possible I try to retain at least 80% of the original frame. 50% is pretty much my limit - and the only if the file is really clean. Yes, you can "get away" with radical crops if all you are doing is posting to the web.
Plus, I intentionally crop to maintain a 4x6 aspect ratio for my Youth Football shots, and a 2166x1600 aspect (roughly 4:3) for my maxpreps varsity shots, allowing an additional 5% safe buffer for the print. Personally, I don't like square crops... even if I'm not printing, I prefer 4:6 landscape (horizontal) or 3:2 portrait (vertical). But for posting to the web, you can get a lot more aggressive and tight, especially if you're not maintaining a standard aspect ratio.
For example, in shot #4 (if your shots were numbered), if you're only posting to the web and don't need to preserve a lot of pixels to enable say a 8x10 print, a lot of people here would love to see it cropped even tighter... tilted a couple of degrees CCW to straighten the horizon, then cropped tighter to completely remove the floating arm in front of the kicker.
Overall, I try to preserve the body integrity (avoid amputations on the main subject), keep the ball in frame, keep some free space in front of the subject for them to flow into. But crop tight enough to remove clutter, busy backgrounds, extra players not involved in the play, etc.
I shoot vertical, with a battery grip if needed for comfort, and crop to my targeted audience. My normal "target" is scrapbooks, fridge doors at 4X6, and Facebook accounts so if I get 1200X1800 I am happy. A little bit of "enlargement" to the 1200X1800 for a 4X6 works for this audience with good glass --- Nikon 80-200 f2.8 on a D3. RAW - No sharpening or noise reduction -- pp applied.
Currently "working" a wall cling request for 5 different grade school players and keeping at least 3K pixels on the long side.
