everyone has their own preference. personally I use ubuntu, because it is simple and it works. I see people use debian, rhel, freebsd, mac, and windows although tbh I never really understood why people go to such hassle to make it work on windows.
The 'hassle' is quite simple and understandable when you, like me, don't have any knowledge of linux
And also, installing on windows is no hassle at all. There is a nice installer in which you only have to click next next next to install OpenERP on windows
I do think that when you have knowledge of linux, it's the better choice for OpenERP because it's its native OS as far as I know. You have more control over the product in that way I believe. But I'm not sure if that's the case