|| and "or" (as well as && and "and") have nothing to do with numeric vs strings. They both are simple tests, the only difference is in their priority: the "words" ("and" and "or") are of a lower priority than the ones a C programmer would know (&& and ||).

'or' is safer to use in some cases because it almost guarantees that everything to the left of it binds tighter (gets evaluated as a single expresion) and *then* gets tested. It's really on a case by case basis. A good and consistent use of parenthesis can often prevent making such decisions, as it removes the the precedence ambiguity.