If you have disk space to spare you can install all toolboxes that are available to you.
Another strategy would be to only install the Matlab base and install the other toolboxes as you discover you need them (which you can do with the Add-On Manager). Commonly used toolboxes (in my experience, which is not mechanical engineering) include the Curve Fitting Toolbox and the Statistics and Machine Learning Toolbox.
I personally work a lot with images, so naturally I added the Image Processing Toolbox. Just go through the list and select toolboxes you expect to contain useful functions.
