The pbp website preparation page suggests carrying 5 to 8 spokes.
Really?
Yes, Really.
Although, having said that..... it kind of depends.
If you know your wheels, know who built them [like yourself], how long you've been riding them etc then you probably don't need to bother.
But, as LWB says, they are absolutely essential to carry if you're riding a wheel other than the standard 32/36 spoke count, built into a standard 3-crossed laced wheel etc because chances are you're not going to find them at any PBP control. You'll be able find somebody who has some tools, and some time to help, but if you're turning a wheel with a specific type of spoke then, basically....forget it.
I've been a mechanic on two LELs where people have
had to pack because their [very nice] non-standard wheels have given way and I was unable to help. And even with more common spoke lengths, you cannot guarantee that all controls will stock them, whereas if you can hand over some spokes of the right length to an on-duty mechanic, then you're in business.
For the weight penalty, it's a no-brainer IMO.