Always Welcome: Sims do react negatively if you cook etc in their houses sometimes. When/if they see you, your sim takes a double negative friendship hit from them and they do the "astonished gasp" face like when you walk in on someone in the bathroom.
Also if you throw a party at someone else's house and your sim does not have this reward (and has not been allocated the caterers position) then when/if the house owners see your sim doing the cooking they will come up, do the astonished face, give a double negative hit to the friendship level AND while this is happening your sim is unable to continue cooking. They just stand there. If this happens over and over again (and it can) then you can effectively miss out on party goals due to running out of time.
I guess, though, more often than not, your sim can get away with doing anything in another sims house.
*edited I actually had this happen in my sims own house during a party. Other guests and family members came up to my cooking sim and did the face and the double negative friendship hit because he was doing an 'inappropriate behavior' !!
The sim reacting to the cooking sim has an interaction called "react to inappropriate behavior". I haven't tried to replicate this since.