Source synchronous transmitter constraints

I thought perhaps that I would be better off concentrating on the skew across the tx_data and tx_clk outputs since they are both register outputs (tx_clk is really no different to tx_data in this regard). So by constraining maximum skew, I would be able to constrain my source synchronous output. So I used the following:

set_max_skew -to [get_ports {tx_clk tx_data}] 0.050

This has a skew which I know is impossible to meet, but the timing analysis run as part of the compile process does not fail as a result. The failure only shows up when TimeQuest is run using the GUI and a maximum skew report is run. So this is pretty useless. Also I have seen a comment that the set_max_skew constraint does not influence the fitter.