You are coming across as a well mannered, educated, wanting to know WTF is going on musician. I understand your confusion. The manual for the amp that you bought says to use the SB Switch.
Some guy (me) on the Internet says you do not need to.
Trust me. I would never tell a guy to do something that would hurt, ruin, damage his amp. They are a passion with me. I build and repair them as a hobby. Some musicians even have enough money left over (after drugs and rent) to actually pay me.
This is an ongoing and confusing topic. It gets real hard, on a forum (unless you are a writer) to thoroughly explain all this stuff.
If it makes you uncomfortable to NOT use the SB.....by all means USE it. I am just saying there is no technical reason to use it, and there is NO proof that lack of an SB damages a guitar amp.
The Power Switch is the one that can really stress the tubes. It runs the heaters. But whatever.
NONE of this is worth loosing sleep over. If your tubes go 1000 hours instead of 1150.....who cares.?
Turn your amp on..... 20 seconds will allow ANY poorly designed bias circuit to catch up, then hit your SB Switch. NOBODY needs to wait any longer than that.
Concentrate on your playing and forget we even had this conversation.
Good Luck