As a quasi-Catholic, I will say that the baptism ceremony (I've never heard a Catholic call it a christening, but that may be regional) is much more distinct from the Mass than the wedding vows. The vows are more or less embedded in the service, while a baptism basically happens after the Mass is over and most of the congregants have left. So it's much more feasible to, say, arrive towards the end of Mass and just wait for the mass exodus (no pun intended). Whether it's polite/rude/insert-value-term-here, well, I posted this because I was curious how other people would answer, because I really can't say.