Unless you want to complicate the swing unecessarily, yes. Think about it. If the trailing knee straightens just a little, that hip will rise with it causing the spine to shift toward the target in order to seek balance. Those moves will then have to be undone at the start of the down swing. There are plenty of good golfers who make that move, so it's not like it can't be done, but you risk power, accuracy, complicate the swing, and you risk back injury by doing so.