The rule of thumb for pixel scale is to aim for around 2 arcsec/pixel for deep sky imaging and around ½ arcsec/pixel for planetary imaging.
Ideally, your pixel scale will be half of the value of seeing for your site (e.g. if your site tends to have seeing around 3 arcsecs, then you want a pixel scale of 1.5 arcsec per pixel).
Oversampling slightly is preferable to undersampling, since undersampling gives you "blocky" looking stars (e.g. in previous example 1 arcsec/pixel would be better than 2 arcsec/pixel)!
Also, oversampling can be of benefit if you are running Maximum Entropy Deconvolution on your images.