I think only you can really decide that and it will depend on weather, temperature, whether you're running or walking etc. If you're concerned, carry a pair of lightweight leggings/waterproofs just in case (although many overnight ultras will insist on these as compulsory kit anyway just in case you have to stop etc. See reports from this years Thames Path 100 for good reasons why.)
I've done both and largely it depends on how I feel. I didn't wear more than shorts during the Lakeland 100 a couple of weeks ago. Other people did.