Irritable ATC
As a rule, the niceties aren't supposed to be used, but most GA pilots like the politeness that some ATS providers give their customers.
I know of one chap who constantly greets his pilots with 'Sir' and 'Ma'am' despite being a civillian. Now personally I think that's going over the top, but he just thinks he's being polite and giving 'good customer service' - I suppose you could argue it both ways, but I'm not a fan.
The odd nicety can ease things, such as ATC saying thank you to a pilot who's 'helped' them out, such as doing an airborne relay or making a smart decision before the ATCO has too.
It's up to you, the RTF manual says the use of courtesies should be avoided, which they certainly are in busier airspace, but those of us with the luxury of a little spare space on the frequency may indulge every now and again.
Britkit