I've noticed the same trend @johnjgeddes. On the other hand... I read and comment on a lot of content (including comments) in the course of a day, and my objective is to burn no more voting power in a given day than I can come back the following morning and start over with at least 95% power. In a sense, I like the current structure because if I see something truly amazing I can give it 100%, but if it's just average-to-good it'll get 15-30% and that still makes it feasible for me to look at 50-60 pieces per day.
Which isn't to say that there's aren't a bunch of people out there who aren't just outright fearful of "voting too much" so they sway in the opposite direction.
I would say that few of my posts move significantly after the first 24 hours... but before that they can; perhaps the result of publishing at "odd hours" so I hit many different time zones.
Thanks, @denmarkguy - you made some good points - publishing at odd hours I can try so as to hit those other time zones but publishing at set times also so I don't lose my base. I like your rationale for allotting your voting power - I think I'll adopt that, but don't take it personally if I only vote 25% on one of your posts LOL!!