I was wondering if it was necessary to buy train tickets for daytrips to either of the universities online ahead of time? I looked at the prices and it seems that I can buy tickets for tomorrow for as low as 15 pounds roundtrip to Cambridge. Is it more expensive to buy them at the train station the day of? I don't want to buy tix too far ahead in case my itinerary changes. Thanks.
Yes, buying "Advance" tickets, which limit you to a particular train on a particular day and are non-refundable, non-changeable are cheaper. Traveling off-peak is also cheaper.
If you decide to buy tix the 'day of' remember that bus fares (National Express coach) are generally lower than train fares.