Yes, this is true. And it would be one thing if you were being given a cab for free that was slightly less powerful than you need or if you found a used one for dirt cheap.
But if you're going to be spending $500 for a cab (like the Ampeg 410 that was linked above) then you might as well spend that money on a cab that is intended to handle whatever your MB210 can throw at it. I would say you should get something that has AT LEAST a 300 watt RMS handling (and of course, must be 8 ohms)
You MIGHT survive fine with that 200 watt RMS handling cab or you MIGHT also blow the cab depending on how hard you push it. Why not just spend the same money on something that is meant to handle that power and you know you don't have to worry about it? I also know from personal experience that I tried this theory out (the one that rovito mentioned that +/- 100 watts is fine) and ended up with a blown horn in my cab as a result when trying to push 500watts into a 400watt handling cab. Speed limits are posted for a reason. They may just be a suggestion but they're a pretty darn good one if you ask me. They're not just arbitrary.
Again, I've tried going with that theory that "RMS watt handling is just a suggestion" and it's led to a ruined cab (due to my inexperience with cab extensions perhaps, but that would never had happened if I paid attention to the numbers). So it's up to you ultimately but I would advise that you head the speed limits, especially while you're getting comfortable in a brand new car.
Always takes a few test drives to get completely comfortable. It's like, if your car says it only takes 5W-30 oil, you could put 5W-20 in it and your car would probably run fine, but it wouldn't run at it's optimal and given the prices being equal and supply being available, why not go with what's recommended?. Or if you have a car that is recommended to only take premium gas (like a nice BMW or something), you'll probably be fine putting regular gas in it but after a while, it will take its toll. I mean, manufacturers, as much as we hate to admit it, do know better than us. I definitely view my bass gear like my car in that it's my baby (my bass, amp, cabs collectively are my baby)
You can change your oil every 5,000 miles and be ok but if you listen to your mechanic and do it every 3,000 miles your car will last a lot longer. Those are the sorts of "suggestions" that some people ignore, but I would recommend you pay attention to. I've just recently had a really bad streak of luck involving these "suggestions" that I ignored and now have blown out the horn in my bass cab and my car is in the shop because I didn't change the oil frequently enough. So now I'm kind of at a point where I'm realizing I should listen to these "suggestions" as the manufacturer probably knows what they're talking about better than we do as much as I hate to admit that.
Personally, I think they know what they're talking about when they post these numbers to amps and cabs. I would recommend, as I mentioned, getting a cab that has at least 50 watts headroom but that's just me.
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