Arduino robot is one of the most interesting topic for every Arduino hobbyist. To build a good robot you need a good robot platform, especially all the wheels need to be equally smooth in order to make it always able to walk straight.
When I start build my first Arduino robot a couple years ago, I bought my 4 wheel drive robot platform from a local electronic shop, it was a simple platform, it works but there is a lot of things you need to aware like wheels alignment.
It takes me some times to calibrate my robot micro-speed motors not like my friends who get their good quality 2 Wheel Drive (2WD) Arduino Robot Platform and 4 Wheel Drive (4WD) Arduino Robot Platform from internet. Once that put everything together they are almost well fit.
In case you are not able to make your robot to walk in a straight line, you still can apply difference speed on both side of the wheels to make it walk straight.