Its always a good idea to do a backup before you update.
Sometimes updates OTA or even updates via itunes fail and then you will need a full itunes restore to get your phone up and running again.
If you don't have a recent backup then you lost everything that was on your phone up till a few minutes ago.
So always be prepared and safer than sorry.
I have my phone automatically back up every night. Over kill.. Otherwise I don't think I'd remember to do it often. I've always used OTA to update, never had a problem. I would assume that is more risky than going thru ITunes . With Good back ups I really don't worry about it to much. I never understand how some people have problems and have no back up.. Couldn't be easier