This post concerns guests with special diets.
I always make it a point to ask potential guests if they have any food allergies or strong food preferences at the time I extend the invitation. This affords me enough time to shop for and prepare a meal that the guest will enjoy. A guest that eats with gusto makes the hostess feel that her endeavors were not wasted. So a little advance knowledge is a win/win situation for everyone in my book. For instance, I don't have to serve a fish appetizer if my guest hates fish. There are other options.
There are some guests though (usually the ones with the most restrictive and complicated requests), who "don't want to inconvenience" the hostess and don't mention their food preferences until very late in the week. Now to my mind - this is rude. Don't come to me Thursday night with a laundry list of off-limits foods and tell me "not to bother".
a) tell me earlier in the week and I will bother or
b) don't tell me at all and I won't bother
But telling me Thursday night with the caveat not to do anything different is disingenuous.
I have mixed feelings about option "b" by the way. If your diet is highly unusual, you run the risk of having nothing at all that appeals to you, and then I feel like a failure, despite your gentle smiles and demure assurances. You are trying to save me the work, but you create a different problem in the process.