Well, if you wanted to power up and get your own Steem Power, you should probably wait until the Steem price is low, near $1 maximum- like right now. It's a good time to buy plenty of Steem, and power that up.

Now, the best time to rent Steem Power would depend on what you are looking for. If you want a high upvote value, wait until the price of Steem rises so you can pay the same amount for less SP, but a larger upvote value. If you want a large amount of SP, You should rent it when Steem price is cheap.