10 C is 50 F
15 C is 59 F
20 C is 68 F
(according to http://www.wbuf.noaa.gov/tempfc.htm)

Thanks all for your good info. I think that unless it's cold (under 10C) I will wear the tri shorts from the start. If it's cold, I don't want to be stuck with cold tights, that wouldn't be nice with the wind factor. But if it's sunny, I don't mind too much.

I don't really see why there would be tri shorts if one wasn't to wear them in the swim. Any ideas?