Yes, you should rebias the amp any time you change the power tubes. It is not strictly necessary in that it will 'probably' work without a problem if you don't, but it might not, depending on the particular set of tubes you use and how hot they want to run. Also, if you have the bias set when you change the power tubes, you can set the bias to the optimum point for sound and function and the amp will sound better this way, and the tubes will generally last longer too. You should also get matched tubes since there is only a course bias adjustment pot instead of one bias adjust pot for each power tube.
Greg
