It has been my expereince as well that they are very picky on even a kg over...so much so that I quit flying them as I often have a lot more than I should (my problem, not theirs but LH is very, very liberal with me and baggage issues).

Honestly, the best bet is to stand in line at check in and watch who goes in front of you. See how strict they are and which of the agents are most lenient. Then move towards that agent. That works well for me on just about any airline actually.

Good luck!