It depends on the person. If you are a skilled trader, being able to convert SBD to STEEM at the right time (and vice versa) can be rewarding.
If you're someone who wants to only spend SBD without ever having to PowerDown SP to STEEM, you might decide to only convert earned SBD to Bitcoin and then USD, etc. (Sometimes its nice to spend a little amount for your posting efforts every now and then and reward yourself)
For those that don't need any funds right now... 100% power up, is the best solution.