Actually, saying most games would be a mistake, and you also don't need to hold B every time to get the option to switch to it.
The majority of first and second party games support 60hz mode (although not all of them -- Wave Race doesn't, for example), but with third party games it's a lot rarer, especially if they're multiplatform. EA for one never utilize the feature, and there are many others. It's really a hit and miss situation as to whether a game will support it or not.
And like I said, you don't need to hold down B every time you start the console. You just need to do it once, and then the option to choose 60hz mode will appear every time until you select 50hz mode. If you select that, you'll need to hold down B again next time you turn the console on to get the option for 60hz mode back.
Hope that helps you in some way.