It is not mandatory to tip the guides and porters because the travel agency pays them. However, tipping displays appreciation for the service provided by a worker. So, you can tip if you are satisfied with the services rendered by the guides and porters.
While trekking in groups, trekkers tip 10 to 15 USD per day to the guide. While trekking solo, trekkers tip 3 to 5 USD per day.
Porters usually receive 5 to 10 USD per day while trekking with a group of trekkers. As with solo trekkers, they receive 2 to 4 USD per day.
There is no set rule on how much to tip your guide and porter. It depends upon you, and on the services you receive.