You aversion to going near/over health and happiness limits is, I think, rather common around here, and I think that people are too afraid of doing so.
To be honest, I don't think there's ever a categorical reason to work a grasslands over a flood plains. Certainly there are situations where that is preferabe (at happy cap, working a cottaged grassland is better than a farmed flood plains), but in general the flood plains is better. What's the difference between the two? A flood plains always has +1

, and often +1

. So, I suppose that there are situations where you think you don't want the food. (you're at the happy limit and don't want to go over, so you slow growth) But lets look at those in specific; turns out there's actually no advantage to working a cottaged grassland over a cottaged flood plains.
The real danger in going over the happiness cap is not that you have an unhappy citizen. Those days ended with Civ 3 (where a single extra unhappy citizen destroyed all the productivity). Rather, it is the opportunity cost of working a more productive tile/using a specialist
instead of gathering food. This is why you might stop working a farmed flood plains to work a grassland cottage, or gold, or iron, or whatever. But when you are working a cottaged flood plains, you are getting the same benefits for no disadvantage. And being pushed over the happiness limit simply does not hurt you.
Indeed, having an extra unhappy citizen is better than being at the happy cap with only 1 food to go before you grow, or somewhere in that last pop. Why? If you ever increase your happy limit, you instantly have an extra citizen available for work. If you ever need a building or unit asap, you have 30 hammers just waiting for use. It's like having a pre-chopped forest available all the time, and the only downside is that you're working a food tile rather than another tile. Choosing between flood plains and grasslands, that's not an issue.
Another advantage to working the cottaged flood plains rather than the cottaged grasslands is that the cottage will grow significantly faster if you're working it constantly, rather than switching between a farmed flood plains and a cottaged grassland when you're at the happy cap.
Going above the unhealthiness cap is even less worrisome. Basically, you lose one food for every extra

you have. This basically means for each extra unhealthiness point you have, it's as if you've turned a flood plains into a grassland. That's why I tend to disagree when people worry about the health implications of growing on flood plains. It doesn't matter because the flood plains make enough food to counter-act the food lost to

. This means that above the health cap, each new flood plains within your city limits gives 2.5 food, rather than 3 normally, or 2 for a grassland. Again, flood plains are simply better than grasslands. That's not to say it's not a good idea to split flood plains between two cities; that's a good idea so more can be worked earlier, but don't do so because you're worried about

issues.
Going above the happy limit can be particularly nice for whipping; according to the old whipping advocates, each

is worth 3

. While I think it's a little less than that, particularly since you can't always whip every 10 turns, and there are other issues to consider...it's still an interesting point. For instance, lets compare working a farmed plains to a farmed grasslands at the happy limit. You might think the farmed plains is better. It slows growth down, and gives you an extra hammer per turn. On the other hand, the farmed grassland gives you +1

per turn. So lets say that it will take you, 20 turns to grow at a +1

surplus. You grow, have an extra pop, and whip for 30 hammers. Then you have 10 turns of unhappiness, where one of your old citizens is not working. At the end of that time, you're back to where you started happiness-wise, and got the same 30 hammers you would have gotten working the farmed plains tile. So, if it'll take you less than 20 turns to grow at +1

, then it might be worth getting the food. If you can get more than +1 food for each production sacrificed (working a plains as opposed to a flood plains), there might be more advantage to the food. This isn't always the case, sure, as there might be commerce in the plains square, or you could use a specialist, but it's food for thought. Whipping more than 1 pop also improves efficiency (as it tends to do), and being over the happiness limit means you're larger, and can therefore whip more pop. Remember you can only whip half your current pop, and that calculation doesn't care whether it's happy or unhappy.
Anyways, that's mostly mumbling. Basically I think people worry a bit more about the caps than they should, and you ought to be willing to go over them if you have a decent reason to, or if avoiding going over isn't worth it.