Market Share by Net Applications is an excellent resource for web designers and developers. They track trends and usage both of web browser applications and of operating systems and resolutions – information that is invaluable in determining design standards.
As a designer, you have to make standards choices as to what trends you follow and what systems your final product will support. Depending on who your audience is, what your company identity is like, and how much time and money you want (or have) to spend, there are a myriad of levels to design standardization.
Maximum accessibility through design and cross-platform / cross-resolution support is always the ideal, but there are reasons why a designer might choose not to pursue all ends to their fullest, and as long as your reason isn’t, “Everyone should just upgrade or switch to my favorite standard to make design easier for me,” there is no true wrong answer.
Always remember, it is you as a designer who bends and works to bring information and products to as many people as possible, not the end user who bends to your needs. Do your best to provide the greatest amount of compatibility achievable whatever the need or circumstance of your client.