I normally rate when I've finished the first season (or given up) and adjust up/down with subsequent seasons. It would be nice to rate by season, though this does seem to be worked out on an average of episode scores.

@YouShouldKnow said:

Season 1 was quite OK, but I'm afraid of giving the series itself a high score as the next season my be utterly bad

I believe the average rating for the episodes for the episodes in the season will be the season rating, it's a bit fiddly tho