If it’s your only alt, then yes you may want to get a boost. However, I have been able to level a couple of low-70s alts to 80 in just a couple days.
This seems to me to be a clear indication that you realize that you are the one who decides what you care about, which follows that you’re the one who can answer whether it’s worth boosting at any time at any given level.
The one thing to keep in mind with regards to boosts is that it seems to bork professions, so if you care at all about having professions, then it might not be worth boosting at all.
Don’t get caught up with staying ahead of the curve in general, I certainly wouldn’t pay 60 dollars to get 5 levels to save yourself a few hours, but even if it was saving yourself a week it probably wouldn’t be worth it. Enjoy the game, enjoy leveling, play it at whatever pace you want. If the pace you WANT is to be 80 the moment cata drops and raiding on day 2, then sure, go ahead and boost. Id just say make sure that is what you really want and its not some other external things making it seem like you need to stay ahead of the curve/ FOMO type stuff. Make sure the time spent in game is fun above any of that.
Dont boost. You wont learn your class as well (or at least as quickly), and you might not feel the same connection to the character. Rogues? You’re a shaman and this is Cataclysm… they’re going to harass you and eat you alive regardless, even if you were 85 and in full set of S9 pvp gear.