I don't think it's rude, but I don't do it - which isn't to say I can't post about my shows, etc. I just use a more cryptic method. Examples:
Can't believe who won DWTS - what a shocker!
Hated tonight's episode of Grey's Anatomy. Anyone else think that ending was totally bogus?
I do it even when I watch things days late, as just about no one I know watches "live" TV anymore, so I never know who in my circle is caught up on which shows. My cryptic comments are enough to generate the discussion I want, and for people who haven't seen it yet, they know enough not to delve into the comments. But this way they don't accidently see the spoiler (at least not from me) when they go through their newsfeed.
I think one time sporting events/championships are different, and maybe even the finale of a competition (DWTS for example - even though I still wouldn't spoil it). And I think if you need to watch something like that on delay and you don't want to know the results, the onus is on you to not go on FB until you've watched, cause likely the whole world is talking.
This also holds true about freak weather. If you don't want it spoiled that there's a blizzard coming in May, you should avoid FB. LOL