The most logical course is to power up as soon as you acquire your Tokens, once staked the new Hive Power is immediately earning interest paid in hive, and augments your Voting power.
However I tend to save some for Hive Power Up Day for the sake of participating as a community. There are often prizes like Hive, H-E Tokens, etc. given to participants if you have less than 8k HP and under Reputation 70 so make sure you use the tag #hpud or #hivepowerupday to qualify. Keep an eye out for #hpud posts for specifics.
PS. And I did win a few prizes in the past.