Well...depends on what you're seasoning the beef with? Salt and pepper? Season at any point (I would salt before, taste, and salt again after draining if needed).

Seasoning with dried spices? I wouldn't season after cooking unless you're planning on cooking the beef more. Dried spices have an unpleasant sharpness when raw. If the beef is hot enough you could try to stir in some spices right after they come out of the pan and hope the heat of the beef will bloom the flavors and soften the edges a bit. Some of your spices ARE going to remain with the beef, especially if you season somewhat aggressively before cooking.

Do you pour off the fat because its extra liquid or are you trying to make it leaner? If you're sautéing 90/10 ground beef, pouring off the fat isn't really helping. You need SOME fat the carry flavor anyway.