Venting a boiler while it is being heated is an excellent idea.
I would tend to vent steam for longer than just 3 to 5 minutes. I would wait until you have a few pounds ( 5 to 10 psi or so) of steam pressure before you close the whistle valve.
The longer that you vent, the less that free oxygen will be present inside the boiler.
Do you use an oxygen scavenger in your water treatment?