I don't know about the tendering system on the Star. I will be on the Star in October. I have been to Juneau and the dock there can hold at least three ships (there were three three when I was there). So it is not likely that you will have to tender. On the other hand, everyone has to tender in Sitka.
We did have to tender on the Star in Juneau,the day before they tell you to go to the library to get your tender pass,go right away when they announce it,we did and got the third pass for tender 1,then you are the first off the ship,you will only have to tender if their are other ships at port,there were three so we were anchored out a bit and had to be tendered in,so if you have a early excursion be sure to get that tender ticket as soon as they announce on the PA where to go get it.
I believe that it is first come first served at the dock in Juneau. On one trip we were the fourth ship in and had to tender. It was no problem. It is a short and quick trip. When one of the ships at the dock departed our ship moved to the dock. (That nearly caused one of our travel mates a heart attack when she returned, saw the ship sailing away and no ship at anchor in the harbor.)