If you want to do web development on Linux you should get to know Linux well. The truth is that if an employer uses a web technology that doesn't require windows, they will save themselves some money and give their developers Linux machines.
Such employers will want employees who are productive on Linux, so regardless of your knowledge of a framework or IDE, knowing Linux helps you be more productive, and consequently the employer will be more profitable.
Linux workflows are often different to Windows workflows. You can automate linux more easily with small command line tools, both existing and custom coded by yourself. Knowing the environment helps you to achieve this. One of the biggest paradigm shifts for example is probably "where are things" with regards to configuration files, and the fact that in Linux you are more likely to edit configurations in a text editor than click through a configuration dialog.
Another way to think of it: You are being considered besides 1 other potential employee who is on par with you on language/framework skills, but he knows Linux better.
ON THE OTHER HAND
There are many Windows based web development Jobs, or mixed environments where you might do some PHP and .Net in the same job position. ASP .Net MVC is great fun, and that requires Windows knowledge. IIS can even run PHP applications these days
My final recommendation would be that if you like web development the best option would be to know Windows and Linux very well. Linux has gotten increasingly easy to use, so it's not that much effort to learn anymore. In many cases development is on Windows but the servers are Linux. It would also be good for you to settle in to the web development framework you like the most, and see on what kind of platforms it runs. Then focus on it's primary runtime platform as your specialty.