The sentence sounds fine grammatically & makes sense, the only thing I'd be worried about is that your correspondent might take offence at the suggestion that you think writing to them might be a waste of time. Are you set on using that exact phrase or could you try something like "I'm not sure if you will be able to help me, but..."?
Actually, i'm writing about an exam timetable, and I would like to know if it is possible to make some modifications on it. The thing is that I really doubt it, since it is written on the top of the sheet "final timetable"... I'm trying, but I don't know if it's worth it...
Ok, with context that sounds a bit less likely to offend in that you're effectively acknowledging that they may be powerless to help you, rather than anticipating that anything they have to say will be a waste of your time!
If the whole sentence is sufficiently polite, it should be okay. Example:
"I'm not sure if it's worth writing to you, as I realise that the exam timetable has already been finalised. However, I just wanted to check if there was any possibility that special arrangements could be made for me to take the ECON101 exam early, as I already have a flight booked back to France on the 21st."
or whatever. Or you could go with my suggestion in the last post. Even better might be a phrase such as "I'm not sure if it's worth troubling you, but..." as it suggests more that you're thinking of the inconvenience to them, not the waste of your efforts.