Yep, folks can and do tweak the diet. The danger isn't so much about affecting losses or even about staying in ketosis - it's about it triggering off hunger pangs that might make you crash off the wagon.
It's also about the fact that some people can't cope with the idea that they've 'cheated'. Having just one thing - even diet coke - that isn't supposed to be on the plan may be enough to trigger off the whole, "Damn, I've blown it!" mentality in some people. Dieting is as much about mind games as it is about the stuff you put in your mouth...
Best not to tweak unless you're absolutely sure you can live with the possible consequences. I try to be quite careful about stating the rules - because they are there for a reason. Hope you don't mind me clarifying, Starr - cos I know lots of people who 'get away' with tweaks too. Including me.
I'm just doing the belt n' braces stuff for the newbies.