In answer to your question about mid-March, I have not been that week but I know someone who went last year during that week. It is right on the cusp of Spring Break, but not quite there yet - so as far as crowds go it will be considered mid-season - not quite low season and not quite high.
The person I know who went last year got rained on most of the days. Which partly leads back to the title of your thread "good time to go?".
There are many people who equate "low crowds" which "good time to go". But there are others (like me) who don't. Yes, I would prefer lower crowds, but in the bigger picture I do not like the other things that often go hand-in-hand with low crowds.
- Numerous unavailable rides that are being refurbed
- Limited shows and fireworks
- Limited park hours
- Greater risk of poor weather
For example, it appears that at least two of the headliner rides at DL will be closed in March - Splash Mtn and POTC. If we go only once every one or two years, I want to go when I know these rides will be operating. And that usually means periods of larger crowds.
See "HydroGuy's High Season/Off Season Tradeoff Rule"
www.disboards.com/showthread.php?t=1199852
In any case, whether crowded or uncrowded,
Disneyland is a great place to be and I am sure you will have a great trip no matter when you go.