The best time to get Steem Power is right now. And tomorrow. And the day after. And the day after that. And so forth.
Steem Power gives you more influence on the Steem blockchain, it gives you a great interest rate, and you can just lease it to someone if you don't want to use Steem for a while. So the best time to get Steem Power is anytime you have money.
And yeah, the price for Steem is low now, so it's a great buying opportunity.