Should I block incoming connections from "netbiosd"?
netbiosd is responsible for interacting with NetBIOS networks. NetBIOS is Microsoft's networking service. 

If you block incoming netbiosd connections then you will not be able to share drives over netbios which is the simplest way to share data to Windows machines.

If you generally block incoming connections as a precaution the you should block anything until you actually need it, that is until something breaks. So yes block netbiosd to begin with.