As with Solutions initial Natural Odour Neutraliser, for optimum performance I would use it undiluted, either as an air treatment or for direct application. Used as a tank additive to perfume the rinse, then 'dilute to taste' applies.
Personally I do not fragrance the rinse as this will have most effect wherever the machine is located - outside the building!
Big company's like Procter & Gamble spend millions on fragrance research (and is why they sell so much Febreze Steve hehe), like JB says most people associate something clean with a fresh smell. It's why you see every supermarket cleaner with a fragrance even though they clean rubbish, it's psychological and built into us. The cleaning ability can be secondary its that powerful.
I remember reading an article on a big bleach company that went odourless with it's bleach, sales plummeted so much it was binned.
p.s Nick has upped the smell on lemon spm from feedback