My feeling is that something will need to be done about this issue to expand WeBWorK to community colleges and high schools. It's a common question/comment that comes from my faculty here using WeBWorK: "I wish it was more clear about decimal precision". The students are generally less aware that a lack of precision could be an issue in the first place, and orientations and reminders don't help. You can add instructions to every single problem about what precision to use, but that is very difficult to maintain. And personally, I think that adding those kinds of instructions to the problem distract from the real math that a student needs to think about. It's why feedback messages from Math Objects are so great, because they can play the role of providing the formatting information if there is a formatting issue with student input.
There is precedent for such messages, even when it's not exactly the source of incorrectness. For instance entering an unreduced fraction in Fraction context or a factored polynomial in LimitedPolynomial context. What if the message was just a reminder, that stressed maybe this is incorrect right now because of a lack of precision? And it could only appear if no other feedback messages had been created. Perhaps a .conf option would control whether or not to give such a message. I'd like that over a command to call like NoDecimals, because it would be all in one place, instead of maintained in each problem file.
It appeals to me to just scan the student answer for decimals that are not precise enough, because that could be done for Formula answers without involving test points. I'll add this to my list of experimental feature branches and if I come up with something that looks good I'll submit a pull request. I will probably ask you to take a look at my regexp for doing the check when I get around to it.