There is actually a window for the driver (or, if you think about it it would have been a bit hard to drive it!)- the Cromwell had a driver vision setup similar to the Churchill with a thick glass plate set into a circular frame (plus periscopes for wider views). This site has some good illustrations of the interior and the various crew positions including the driver's and shows the driver's view;

https://www.kithobbyist.com/AFVInteriors/crom/crom.html

Depends on the degree of realism you are going for- in Op Flash they model the 'heads out' and 'interior' drivers views where applicable, according to the model of the actual vehicle. But it depends on your audience. In realistic games one of the issues in driving tanks is coping with the restricted vision of different models etc. But being an armour freak I would say that 8).