I call it the squeeze. A writing job entails a list of things you have to do to complete the task and earn the agreed upon rate. The squeeze happens when clients add little extras in. I'm not talking about the customer expecting a revision if your finished product didn't meet expectations,I'm talking about having to do extra work above and beyond what the rate and original work brief called for.
When the squeeze happens, as a writer, you have to decide whether or not it's worth holding onto the job. If the job will bring more work, more exposure or is a great rate, you might consider accepting the extras as part of the job but if it's just more work, you might need to think twice.
Also consider the fact that if the client is squeezing more work out of you for the same rate today, they are likely to try to do it again in the future. There are big clients out there that hire many writers that slowly squeeze more work out of writers. It starts with a little extra and then before you know it you're doing fifteen things to earn that $10.00 instead of ten things.
If 500 words for an agreed upon rate turns into 550 or if that word document suddenly requires html or xml formatting, think about your time and efforts. While you may not want to lose a gig, you are now being paid less per hour for your work efforts. There doesn't seem to be a code of conduct or employement standards and ethics for freelance writers. If you were on payroll with a company, you'd expect overtime or lieu time. They would have standards they'd have to meet in order to be law abiding. As a writer, try not to let the client away with treating you like you work in a sweat shop. If enough writers stand up for their rights and principles, fewer clients will try to get away with it.