Yes it will decrease your steem power because steem power will be increased and decreased based on your up votes as you up vote 100% to any one i guess 2% of steem power having 500 steem power account decreased almost. So i have tried this many times and yes this decrease your steem power.
Because blockchain is connected with each other of steem and musing so for sure it will decrease your steem power by your voting status whether you vote 100%.