College applications: Where does "recommended" end and "required" begin?

<p>Northwestern “recommends” 3 subject tests, and I’ve taken 2. The only (and I mean only) other subject test I’d feel comfortable taking is Chem, but that won’t be until October, when I will have forgotten everything. Should I waste my time restudying a whole year’s worth of material or just submit the 2? (I did really well on the 2 I did take; Lit-740, and Bio-M-780)</p>