IDL uses three font systems for writing characters on the graphics device, whether that device be a display monitor or a printer: Hershey (vector) fonts, TrueType (outline) fonts, and device (hardware) fonts. Fonts are discussed in detail in Fonts.
Both TrueType and Vector fonts are displayed identically on all of the platforms that support IDL. This means that if your cross-platform application uses either the TrueType fonts supplied with IDL or the Vector fonts, there is no need for platform-dependent code.