In the majority of cases, talking about food is not a problem at any time. The few situations where it might be considered impolite would be when you are speaking with a person who has specific rules about what they eat about something they feel is inappropriate to eat. For example, some vegetarians might not appreciate talking about eating meat.

You can talk about food anytime. Examples of that could be that you had a delicious meal at a restaurant, giving someone a good recipe you know, etc. However it would be inappropriate to talk about how bad the food might be, if you were invited to dinner somewhere, like friends, co-workers, bosses, family, etc. It is an insult to them.

It's not considered rude where I am from. Also, I've been to different countries and I have never heard of this.

Its great that you care so much to worry about these things Ekk. Don't worry, if it is obvious that you care about the other people then they will never mind because they will know it may just be a cultural thing. At least in larger cities in America we are all used to having a bunch of different cultures and in many ways have embraced our differences. Don't worry!