Agree with you Pruners. Know lots of people who started off demand feeding, then moved to more of a routine to fit in a bit more with the rest of their lives.
None had any problems with supply at that stage.
However, my ds was such a sleepy baby that if I'd demand fed, he'd have slept for hours between feeds, and I don't know what sort of mess we'd have ended up in, if I hadn't followed a 'well known' routine. As a first time Mum with a baby who just wanted to sleep, it was a godsend to me, as I knew his needs were being met, even if he didn't think he needed it IYSWIM.
Ds took himself off the routine at about 10 weeks, I didn't force the issue, and a new routine evolved that suited us both.
We haven't done the routine since those early days, but I'm convinced it put us on the right track initially. Ds has always gained weight well, apart from taking 3 weeks to regain his birthweight ( when I was following his lead and demand feeding.....) and slept through from 4 months. I never had problems with my supply either.