Text entries in the spreadsheet serve a descriptive purpose. Text can be used as row and column headings and to describe particular cells. In fact, as soon as you press one of the alphabetic keys (letters of the alphabet), Calc knows that you are making a text entry in a cell rather than an entry of numerical significance. So, Calc won't try to perform any math on cells that contain text.
In a number of spreadsheet software programs, text entries are often referred to as labels. Calc really doesn't use this term to describe text entries. In Calc, a label actually consists of labeling of specific area of a spreadsheet with a name and then using that name for navigation and formula-writing purposes.