While they often don't leak, the instructions say to continue to tighten it. If you first ensure it is even side to side, then tighten about 1/2-turn alternately on each side, you'll be able to get it tight. I like to leave a little play, but just barely. You don't want to get it tight and then try to compress it...porcelain is brittle. But, it is plenty strong enough if you tighten it down evenly. For a quick guide, you could stick a thick piece of paper between the tank and bowl and then tighten until it just starts to generate some friction and you can still pull it out.
Now, whether I'd call the plumber back or not, depends. The instructions do say it should be tighter. It's an easy fix, but if you break it, you've got problems. Shouldn't happen unless it is either defective in the first place, or you overtighten it. As long as there is a gap (you can pull the paper out in my example), it shouldn't hurt the porcelain if you do it evenly side-to-side (put a piece on both sides near the bolts). It's easier to tighten it down if you lean on the tank while doing it.