It's worth to wait the right moment to exchange SBD for STEEM
When SBD was at $11 while Steem was at $2 was a good time to exchange.
Had you exchanged them 1:1 a few days prior, then you wouldn't have made the same amount by author/curation rewards.
Of course SBD is supposed to be pegged at $1 USD, so their are no guarantees.
IMHO, it's as much about the ratio between Steem and SBD as it is about the USD value. You're right, that it's better to exchange when the SBD value is higher in relation to the Steem value, and I do look for that. Honestly, I sometimes just power up so I'm not tempted to spend the Steem. :)