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The answer to this question is nuanced.The basic answer is: no. No "no" but there may be reasons to try to stay within the Ed-Fi-defined set.
First, no one is forced to use the Ed-Fi values. The Ed-Fi technologies and specifications are open source and anyone can do anything they like with those resources, including manipulating descriptor options. Plus, the Ed-Fi Alliance and its community have endorsed the notion that there is no set of code values (for example, for grade level) that is "universally applicable." By definition, this means that we need to expect that code set will be localized.
However, just because variation will always exist and just because there is no "universally applicable" set of values for any concept, it does NOT follow that there it is not value and good reason helpful for us to adopt a common vocabulary, and for the Alliance to encourage - sometimes strongly encourage - community members to adopt common sets of values for particular concepts. When we act in common ways, then we can rely on each other, share resources, and interoperate with each other easier.
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