Now that I have your attention, there are some cases you can do either or both. Some lith developers use formalin and some non-ammonia fixers use formalin. In these cases, remember that any amines or ammonia present will react with Formalin, so Metol will react as will a PPD developer. Ethanol Amine and Diethanol Amine will react with Formalin. Ammonium Hypo will as well.
If you are going to play with this, you have to know some chemistry.