I've never made any deal about sweets at all.
We have ajar on the worktop in the kitchen where sweets go - ones that arrive for various occasions etc, and DS can help himself when he likes. He also has a tin in his bedroom of birthday sweets. I think he helps himself perhaps twice or three times a week, to one or two sweets.
DP buys sweets at the cinema (I never do) but mostly they bring most of the bag back home.
Once when DS was a toddler he was eating spag bol (he was a DREADFUL eater, small appetite, little interest in food, fussy) and spotted a bag of choc buttons the other side of the kitchen, and asked fro one. To the horror of other parents present, I gave him one or two buttons, he ate them, and then went happily back to his spag bol.
I think this is partly down to making no big deal - have never used sweets or puddng as a bribe or treat or something to be earned by eating savoury food - and partly down to intrinsic character - he just isnt obsessy or insistent about things, and partly down to not being hugely interested oin food anyway. He eats a fantastic range of foods now, very adventurous, but will have seconds of a savury course he really enjoys rather than dessert.
I had to stop leaving sweets out when his friends came round as they went mad raiding the sweet jar, but now they leave it alone, too - will maybe ask to help themselves to one, i say OK, and that's that.