It depends really. If you are planning on housing them together for a while then it's best to do it now. The 'popping' method is probably best for young snakes and you can also count the tail scales after their first shed although this is not too accurate. This is all just to ensure that they don't attempt breeding too early.
If you are going to house them separately, however, then there's no rush to get them sexed. I would wait until they are much bigger and then go and get them probed.
Hope that was of use,
Neil