It seems to me that every new coin introduced, they list source code in Github. Is that customary in the industry?
Most users are not going to trust a closed-source altcoin, so you should make its code available to the public. This board also doesn't support closed-source cryptocurrencies.
GitHub provides free hosting for open-source projects and has become a standard in the open source community so (primary opinion-based statement follows) it's probably a good idea to put your code there.