I'm wondering if airlines usually allow as a courtesy a certain amount over the stated checked baggage allowance, and if so how much is customary?

I'm flying Batik Air soon from taipei to KL, and their luggage allowance is 20kg.

If my luggage ends up weighing, say, 21.5 kg, if they are likely to let it slide, or is it advisable to always try and remain under the stated allowance to avoid fees?