I seldom crop my macro picture for magnification (make it big purpose) but I do crop for few reasons
1. make it little out from center ... not sure why but most of the people don;t like too center ... make be this is call rule/art ...
e.g. of crop one
and original uncrop version
2. make it center, for some situation e.g. some symmetry shot need to be center so minor cropping will help to do this
eg. this had been crop ~10% from left so to make it over center to make some attraction
3. this is the one keep border me where I am using FF. to get same DOF of same magnification with APS-C body ... I just need to crop (FF 1.6:1 ~= APS-C 1:1 and bigger magnification will have less DOF this had discuss @http://liewwk-macro.blogspot.com/2008/10/what-depth-of-field-dof-you-have-while.html
not only the magnification but as the diffraction concern, I normally use bigger aperture (smaller F number) to shoot bigger magnification to maintain the sharpness so I do lost more DOF while shooting bigger magnification as compare to APS-C (but actually I got the similar frame picture)
shot ~3 :1 without crop
shot with 1:1 and crop close to ~3:1
so should I corp ? or should you ?
it is depend personal preference ... I do not prefer to crop but stack so I shoot multiple focus and stack it together as discuss @ http://liewwk-macro.blogspot.com/2009/12/macro-pp-why-i-need-to-increase-macro.html
and you ? just shoot and enjoy whatever way you enjoy the shooting and the output instead other concern ...