If you have a multi line taskbar they will go to multi line without shrinking as thats how windows works.  It would only start to shrink when there is no room for any more.

Likewise they group themselves based on the number of any one app running and based on the space available.  There should be no difference between this with WB running and without WB running other than a larger start button or tray area will naturally reduce the amount of room for taskbar buttons and so could cause them to group quicker.