Entirely up to you - the law doesn't define anything but the absolute minimum information that must be in your logbook.
You might, for example do some glider, helicopter or microlight flying - in which case the spare couple of columns would be ideal for the purpose.
On the other hand, although it doesn't legally comprise part of your total flying hours the odds are you'll manage to scrounge some interesting and educational passenger flights as you go along. Many people use a spare logbook column for these flights as well.
For that matter, there's nothing to stop you recycling other columns (such as multi-engine) which you are unlikely to use for a few years.
G