Sometimes those 5 gi will make just the right amount of difference, other times it will barely matter at all. Adding a tooth will put your axle about 1/4" further into your dropuouts.

I recently switched from 49/17 (76 gi) to 49/18 (71.8 gi) because my left knee has been bugging me some and the slight difference was just what I needed for the time being.

It's good to have a bunch of different size cogs on hand.