FYI you can sell your SBD on the open market instead of waiting a week to convert it to Steem (use the buy/sell option and you can either accept someone's bid or make your own and hope someone takes it for a slightly better price than is available). I personally chose to power up all the SBD I earned immediately because I wanted to reap the benefits of higher reputation score, more curation power, more money given to people for votes, and as you said you get a better bang for your buck on vests by buying them earlier.

If you are worried about steem price fluctuations you could always spread out how much SBD you spend on steem per day. For instance, say you had 210 SBD in your account and wanted to convert it to steem to then power up.
If you sell 30 SBD per day, you end up buying more steem when it's cheap and less steem when it's expensive (because you trade in a fixed amount of SBD but when steem is cheap that buys you more and when it's expensive that buys you less). this averages out the swings and keeps you powering up steadily.

But now there is an added wrinkle: post promotion. I want to allocate some of my SBD towards promoting my posts and some towards powering up to invest in more followers which pay dividends over my future on the site.

It's an interesting question and the answer varies person to person depending on their financial safety net, their belief in the site long term, and how much time they want to commit to steemit in the future.