All depends on how fit you are
If you are the average couch potato looking to get fit 10 Kg above is OK and you will probably be fine on them untill you wear the springs out anyway
If you are fairly fit (find a building with 5 floors and run up the stairs) if you make it without stopping your fairly fit you can then probably go for either 10 or 20 kg
Remember though that you are at the bottom of the weights so for you 10 Kg is 1/5th of your body weight and 20 Kg is 2/5ths
People that are in the higher range say 100 kg are only going up 1/5th by going 20Kg up so you might think about it that way instead
Now if you are super fit (can flip on your feet and run up the 5 flights of stairs while giving a mate a piggyback
) you can go as high as you like but try some first
Jason