It is my understanding and remember I am not the sharpest tool in the toolshed...

But every month you get paid interest on your SBD and SP and you get nothing on your liquid Steem . . .

Over three years your Steem Power bleeds away and reduces by 50 % at the 3 year mark. This means if your a Whale with 1,000,000 in 2016 and do nothing then in 2019 your account will have lost fifty percent and dropped to 500,000 SP.

However who cares ? , if you use Steemit for whatever reason you will earn something sure it might not be a thousand SP every week but over the weeks it does add up... (Assuming you are actually being respectful and trying to become a better craftsperson presenter on Steemit)...

So to my mind always be powering down, PD occurs from when you make the request each 7 days. So if you suddenly have an extra 2000 SP and are desperately seeking liquid Steem just wait till your next power down occurs and then make a NEW power down at the higher SP rate.

But generally I think people should be powering down always and then if they do not want nor need to sell or transfer that Steem they should just immediately power it back up.

The more you have in SP the more you earn as rewards based on your number of vests, but sometimes depending on your situation it might be useful to keep 5% or even 10 percent of your Steem Power as liquid Steem. Sure you will not get interest paid on it but if your into cryptocurrency trading you might make a nice profitable trade by keeping Steem immediately available : )