We bought over our allowance, and put in a few warm items just in case, and lots of hot weather items and stuff.
A good point to note is, check the small print on the airlines website you are flying with, as for example, we flew with singapore, and in addition to the 40 kg per person plus 8kg handluggage, we could ALL also take on and umbrella, a laptop, coat (if you have a goretex, put the fleece liner in), and a few other items which could lighten you case a little.