Hello Charlie!
Not a doctor by any means, but my understanding (from hearing about others and their 'travels' through the medical world) is the best time to have an MRI conducted is when you're experiencing a flare up of symptoms/relapse.
It does make sense, my first MRI was before I was diagnosed, and I'd massed a TON of different symptoms over the span of several years (some came and went, others stuck around). When I had my MRI, the lesions lit up like a Christmas tree.
*Important: Be sure to have an MRI done with "contrast" (it's a dye that they inject partway through the procedure). This way, your non-contrast MRI results can be compared with those which involve the contrast. The contrast will show active areas of inflammation/lesions
I've had about 1 MRI a year, and beyond my first MRI, none had changed (first was done late 2007/early 2008, I was diagnosed in March of 2008) until my last MRI which was done late last year (2013) which showed 1 old lesion that was active again, and a new lesion almost right on top of the older lesion that had re-activated. The new lesion was also 'active' as well.
Although I had active lesions showing, I didn't have to take anything (solumedrol or prednisone, for example) to reduce the inflammation. I guess my neuro felt that, based on the results, if I wasn't experiencing symptoms that had me knocked out for the count, best to wait it out. I agree though... many folks won't ever do steroids and such for relapses, others will for a major relapse. It's all down to personal choice. Keep that in mind in your journeys.
To my knowledge a lesion doesn't "heal" so to speak, a lesion can become inactive though (remission). If you're thinking of an MRI just for the sake of seeing if a lesion that was located on a previous MRI while you were experiencing L'Hermittes Sign has gone 'inactive', I say no it's not worth it. If you experience a new neurological symptom that's quite bothersome/troubling, that's when you should be speaking with your neurologist/GP, and perhaps then an MRI would be a good idea.
Always, always inform your GP/neuro of new symptoms that come up, and ask them if further investigation is merited. Depending on where you live, an MRI may be expensive (and the result may not show anything new, so money spent for a repeat, you know?) or may be a long wait to have done (I'm in Canada so MRI are covered by medicare here, but the waiting list for one can be quite long, luckily I got in last time on a cancellation).
Again, speak with your doctor for the best advice for your situation. All the best.
