Is storing values of a unique field shared between many web forms bad practice?
I think that what you're really asking is twofold:


  Is it general practice to make a table with a few custom fields that are accessed by many pages in the app specifically for those few custom fields on many pages.


While it's unusual, this is rather part of the whole intent of having databases in the first place. A central location to store data that can be retrieved from consistently and easily, and that is abstracted from the design of the site.


  Is it going to cause performance issues to have [many of these tables|tables with such small sets of data]?


No, not at all. Database engine designers know that these things will happen, and they're usually smarter than you or I. We can discuss this more if you like, by discussing the mechanics of database engine design, but it should be sufficient to say that "the smart guys thought of all this already".