To me there are 3 main categories (apologies for how small the pictures are)

Helmets who's basic design is not interrupted by the addition of required sponsor logos either in design or colors used

interrupted or 'ruined' helmets where the introduction of sponsors either drastically changes the flow of the design or in some cases (like Kimi Raikkonen's new Mclaren lid) require a new design entirely
and

The driver who designed his helmet to include sponsor logos to begin with. Granted in this case Panis modified his helmet, but the en vogue thing amonst young drivers currently is to include the white horizontal band around the top of the helmet in the hopes that when they're in F1 it will be filled with something important.
So do you guys know when this shift really started? My own helmet design isnt sponsor inclined, but when I drew it out I kept conversion in mind. Ie if I get to a point where I need to carry logos in certain areas, it wont disrupt it, and it doesnt look weird without them either. Took me a long time to come up with a compromise

I notice in general that road racing helmets tend to be very geometric and solid both in color and in design, compared to say bike racing. There's also, I feel, a tendency in helmet design depending on where you are from. Japanese drivers tend to lean more towards the bike crowd with different colors, lots of shading, and blending. Italian karters often have really elaborate multi-layered curved lines and neon colors.
Discuss amongst yourselves.