There are only two rules where hailing is mandatory, in rule 20 and in rule 61.1 of the racing rules. For 'room to tack' approaching an obstruction and 'protest'.
If you want to know why Dick Rose's count ends up at five, you will have to read: Five Important Hails
A small quote from the piece:
“Tack or cross?”
In many fleets these days, the hail you hear most often is, “Tack or cross?” It’s most often made by a port-tack boat on a windward leg that would like to cross in front of a starboard-tack boat, but isn’t sure she can do so without forcing the starboard-tack boat to alter course. The hail is shorthand for “Waive me across your bow and, if need be, bear off to let me cross.