Composition is very important in how the overall image looks, sharp or not, and while it is preferred to frame for the end product from the beginning you do have a bit of 'canvass' to work with in the higher mp bodies out there. Do not be afraid to crop into the image, that is one of the benefits of digital photography, but you want to make sure you are capturing the sharpest image you can if that is your objective so you can maintain sharpness in the crops.
For example my normal wildlife lens is in for its yearly service (70-300mm-L) and for the past several days I have been playing with the super cheap 70-300 III kit lens from a friend. Not very sharp at all in comparison, but stopping down the lens to its sharpest (f/8-f/11) and then purposely choosing a ISO setting that starts to every so lightly show grain can give a crispness to the image. That will give you the sharpness you may need to perform some PP composition and frame the subject for the least distracting background/surrounding.
(ISO 640 probably showing a tiny bit too much grain with this lens.. and if the branches in the right side of the frame were not so distracting it would have been better to have the squirrel further to the left of a slightly larger perspective.)