This is a very personal issue -- but in the context of knowing more about her SM, or for research, as Margaret says, most of what could be used or discovered in direct relation to cavaliers and SM would need to have been done close to the time of death (generally within hours and the closer, the more helpful for research) and arrangements in place in advance.
For anyone considering this and wanting to have this possibility in place (it cannot always be arranged in the US/outside the UK/Ireland, where extras time is likely needed to get arrangements in place -- a vet school or neurologist could do the needed tissue samples and analysis) Margaret has posted information over the past couple of years here on the site and there is detail on Cavalier Matters and also Carol Fowler's Cavalier Campaign site. This kind of discussion, though painful, may help others to decide that they would like to make such arrangements so is important and helpful for that alone.
If Dr Shores would find an autopsy helpful for his own information and if it might give you some comfort to have any more information, Linda, then I'd have an autopsy done. But as others say, you will not really get any of the information about syrinx development and internal structures that would appear on MRI while living, or that might possibly be obtained within a few hours of death, since the syrinxes collapse and there's no evidence of them. You might be able to get information on any shifts in the structures Dr Shores put in place with the surgery (though this is very unlikely to have happened) and I am sure that any scar tissue development would be evident (which might help in understanding how things worsened). An autopsy would also give information on her heart and evidence of MVD/fluid accumulation -- though again given the time that has passed, I am not sure how much can be understood. I'd have a detailed conversation with Dr Shores about whether this is truly something that would produce any helpful information, if you are at all unsure of whether to have an autopsy done.
On a personal note I'd never put an older cavalier with advanced SM through a spinal tap. I wouldn't do this on a younger dog either unless it was absolutely necessary.