Depends a lot on the variety. Here in the U.S., Victoria has a bad habit of flowering and seeding early.
If this is the first year for your planting, you shouldn't harvest it anyway, since the leaves provide nutrients thorughout the summer season. If, next year, your plant sends up flowers and seeds, you can certainly cut the seeds off as soon as they appear and it won't hurt the plant and you'll be able to harvest the stalks for use...
You may wish to check here for one source of information:
http://www.rhubarbinfo.com/growing