How much does it matter the mentioned intended major at grad school in GRE registration?
I wouldn't worry about it because that portion is used for statistical purposes like compiling average test scores by major, like in the example here: http://www.ets.org/s...uide_table4.pdf 

The information that schools look at that regard majors are if you take a subject test, like in computer science or physics. And of course all of your undergrad data. 

Additionally, marking undecided is possible if you really dont want to choose one.