Is it fraud for my supervisor to ask me to sign my time sheet a day ahead so she could send it in on the day they are supposed to be submitted to the pay office?
Short answer: If they are the hours that you intend to work and do work, it is absolutely not fraud.
It's very common for this to happen. I've done it at least a dozen times in my career. Sometimes you'll be working off site, or for some reason unable to get a timesheet signed on the day they're normally submitted.
If for some reason your actual hours vary from the timesheet hours you submitted, you can just notify them of a correction and if necessary any adjustments to pay.
If the supervisor gets a bonus for collecting and delivering all time sheets quickly at the end of the day, and gets his bonus by delivering time sheets that were filled out before you actually did the work, lying about this to his boss, yes.
If you are forced to work an hour longer and the supervisor denies payment because that hour is not on your time sheet, yes.
If you don't arrive at work because of an accident, and the supervisor still charges a customer for the eight hours on your time sheet, yes.
As long as all the data on the time sheet is considered as preliminary and is adjusted to reality when necessary, it's fine.
The company has to have in place procedures for how to address changes between the time you sign, and the end of the pay period. If they are following the procedures it is not fraud.
If they are billing customers for your time the customers will insist that either time cards can only be submitted after the end of the pay period, or that there are documented procedures for addressing these changes. They will want to be able to see not only the final time card, but the list of changes and the reason for those changes.
Besides being able to bill correctly, which does allow them to keep customers, the employee also wants to have these adjustment procedures to protect their vacation. For example if you think you will be on leave the last day, but you are called in for an emergency and have to put in 12 hours of work: you want to be paid for that time not charged 8 hours of leave.
There should be time card procedures documented, and you should receive training on those procedures. Many companies require annual retraining to reenforce those procedures.