You can opt to tweak them first - in which case you should make sure that your monitor is accurate in showing close-to-lab-print-colors-and-brightness.
Or you can pass a small batch to the lab and ask them to adjust for you (spares you all the trouble if you don't know how to do it yourself).
Or you can send a set of 3-4 sample prints, ask for No Adjustment, and bring them back to adjust your monitor. Then adjust the rest at home (based on the newly adjusted monitor settings) and then send the rest to the lab.
It's entirely up to you.