In mathematics, the typeface says what your variable is. It is like writing pCounter
in a computer program to make clear that the variable holds a pointer.
For instance, one of the many common notations in linear algebra is that capital letters like $A$
represent matrices, lowercase letters like $a$
represent vectors, Greek letters such as $\alpha$
represent scalars, script letters such as $\mathcal{A}$
represent subspaces.
The actual letter that you use often is a "default letter" for that kind of mathematical object: for instance, $D$
often represents a diagonal matrix, $H$
a Hermitian one, and so on.
Of course, the choice isn't always obvious, and often notations for different fields clash (for instance, you may want to use $\delta_{ij}$
as the Kronecker delta, but $\delta$
as the prototypical calculus "small positive number").
Using the same letter in different typefaces is often reserved for related quantities: for instance, you could call $d$
a vector, $D$
the diagonal matrix with diagonal entries $d$
, and $\mathcal{D}$
its spanned subspace (just the first example that crossed my mind). Or vectors and their containing subspaces: $u\in \mathcal{U}$
, $v\in\mathcal{V}$
.
If you use the same letters for different things, I assume that they are either "default letters" or related quantities. Otherwise, you are just making life harder for your readers.
TL;DR: It depends on what your variables represent.