They changed the way Assassination victory gives points.
That being said, you should go for an assassination when:
a) It is a safe one b) There are no other options left
Scenario wins are there, sure, but you can always get caster killed yourself. If you have a lane and the % is decent without putting you at risk too much, go for it for sure. If you are behind and you don't see a way to claw your way back into the game, go for it as well. To be honest, it's better to win with a low CP total then to lose with a high CP total. Victory comes first, CP comes 2nd. Try to see if there is a way to score some points on the turn you go for your caster kill, if not, end the game and relax before the next round!
You should always be looking to score points, and go for an assassination when the time is right.
Scoring points is the best way to put pressure on your opponent without directly threatening their caster. Thus is the most important lesson if you are just starting steamroller: always be looking to clear zones whenever you can, don't leave free points on the table. Do your best to always be threatening to score and therefore requiring your opponent to contest lest you score on their turn. Make sure to keep a jack or a unit within a zone, keep solos on or near flags. It puts a lot of pressure on your opponent and can save you if things go south.
Assassinating should always be done with care - if you are up 3-0 and you have the option to clear two zones or assassinate, you are probably better off clearing the zones to win. Don't throw away a sure victory in two turns for a less sure chance this turn. On th other hand, if it's all ypu have left then go for it.
Realistically, if you are up on attrition then usually the best option is to preserve that, even if you pass up on what appears to be a juicy assassination. Odds are not usually as good as people think for assassinations. It also helps if your plan has good stopping points: a banshee knocking over their caster is a good example - if it hits, go for it! If not, it's time for plan B.