The best time to buy...
... is BEFORE you absolutely need it. After being a graduate student for 6 years, working for a couple, and now a faculty member, computers have a habit of failing right during critical times. If you are in a major corporation and you have lots of IT support, great.
If you're more or less "on your own" and you will be to some degree as a graduate student, an extra $2000 now to get the latest and greatest might protect you against the inevitable failure, drop, or other disaster that ruins two weeks worth of data collection, analysis, and effort.
Your graduate thesis is a one time deal. Always have a back up computer that you can trust and that you know. 
... is BEFORE you absolutely need it. After being a graduate student for 6 years, working for a couple, and now a faculty member, computers have a habit of failing right during critical times. If you are in a major corporation and you have lots of IT support, great.
If you're more or less "on your own" and you will be to some degree as a graduate student, an extra $2000 now to get the latest and greatest might protect you against the inevitable failure, drop, or other disaster that ruins two weeks worth of data collection, analysis, and effort.