Depends on whether or not the states check it for validity and my knowledge is so out of date that I can't say if they do.

You'd have to avoid any SSN which exists in an illegal range and you'd want the first three numbers to be consistent with where you claim to have applied for the SSN (usually the state where you were born - fewer questions). See also: http://ssa-custhelp.ssa.gov/app/answers/detail/a_id/425/~/de... and http://www.usgennet.org/usa/ne/topic/resources/basic/ssn.htm...