My first two MRIs were done with gadolinium contrast agent (GAD). The next two MRIs were done without. There is no problem identifying the edge of the tumor without contrast agent. Actually, it is better at defining the edge since there is no bright tumor image that makes it hard to make out the edge clearly.
My summation is that if you want to see if there is a tumor present, GAD is best. Particularly in identifying small tumors. If you want to do a measurement to determine the growth profile I actually prefer without GAD.
My unqualified opinion is to use GAD for the diagnostic MRI and then never to use it again.
In an unrelated MRI that I recently undertook, the doctor prescribed an MRI with GAD. I called back to ask if the GAD could be skipped and he said yes. According to the doctor, the radiologists prefer GAD, but can work with it.