Marginal differences in rates will not affect the amount of spending money you have, just one drink! I always take a couple of hundred and top up at cash machines if needed. A small amount of leftover euros can be kept for next time.
There is a small bureau in our local Cash Converters store which give the best rate available anywhere round here and we have never had a problem using them.
Do check the amount you end up with as some places that give higher rates also charge commission, which means that you can end up with less money than changing at a lower rate but commission free.