I'm thinking back through all of my many jobs.
Jobs 1, 2 and 3 operated a flexi time system with keys being put in slots to record how long you were in so you could take that time back later. There was a different slot if you were claiming the time as overtime rather than flexi. Overtime wasn't optional - if the project needed more work you had to stay, but it was paid in addition to your usual wage. No-one stayed for free! Jobs 1 & 2 were on fairly low wages (about 15k in mid-late 90s). Job 3 was a lot better paid but the stakes were high so bascially management paid us very well in order to keep us happy and put overtime on top.
Job 4 - had to leave on time as the office was locked at that time.
Jobs 5 and 6. Was part time and flexibility was restricted by nursery/school drop offs. But I was as flexible as I could be.
Job 7 - full time. Very much task led. Sometimes I'll put in 30 hours. Some weeks it'll be 40-45. My DC was seriously ill and I was so touched by how good my bosses were - told me to take all of the time I needed, on full pay, no worries. I think they were so good with me because I do put a lot of graft in, I am flexible to their requirements so they paid it back.
In summary, I don't think YABU to stick to contracted hours, in fact some jobs demand it. However there is a lot to be gained from a mutually flexible working relationship. I think the key is feeling that you too are gettinmg something out of it - career prospects, time back - rather than exploited.