It annoys me a little bit just how often people forget these details, so I made some comparison.
 
Most ponies have oval shaped eyes, but some ponies (for example Twilight) have almond shaped eyes. Most ponies have highlights in their eyes, but some ponies (for example Trixie) doesn’t. There are also ponies like Fluttershy, having special eyes. Drawing wrong eyes may make your ponies look strange. Same thing applies to OCs.
 
Or maybe it doesn’t really matter. In that case, ignore all I said.