If you do end up taking the subway (or just in general actually), have a decoy wallet with you containing MXN100 or 200 that you can give away in case you get mugged. Always a good idea to at least have something you can give "them".
Note that according to the official website line 5 (the line at the airport) is currently being worked on, and that there is no service at all on weekends on its northern half - so in theory you should be OK, as you need to take it "south" from the airport. But you never know...