I put off taking some important classes until my senior year, should I still apply to grad school during my fall semester?
There are lots of programs where important classes are not taken by students until their senior year, largely because of the design of the major. Also, in most cases, if there is a "capstone" course, it's not taken until the final year, and grades would not be ready until after applications are due. However, this has not proven to be an obstacle for students applying to grad school, because the people making the decisions are cognizant of such organizations, and take this into account when making their decisions.

So don't worry too much about applying with your senior year courses missing—most of your peers will be in a similar situation!