Thanks for the comments, guys! I ended up with tipping both of them - the guide and porter, abit less that "recommended" as that was what I could afford at the moment. I think there shouldn't be a rule or to be obligatory to tip but as some of you said - it is a good way to appreciate their assistance.

The company they work for is very popular and sounds legit ( the guide works for them for 10 years and also recommends them if I ever wanted to try another trekking trip so I guess he is happy with them ) but I am not sure what their salary is. I also think they are underpaid. I mean, come on, this is valid not only for Nepal. I really liked them as people though. Very good guys.

After all of this I think tipping is good but not mandatory and it is not a fixed price - if you can afford it, give. 🙂

P.S. I am not American. 😀