Obviously being a teaching assistant as an undergrad or a masters student looks good, but how good does it look? Is it merely a nice little bonus, or does extensive TA experience have a large impact on one's acceptance?
From my perspective admitting students to CS PhD programs:
TAing for CS classes will be a small positive signal that you did well in those classes and were well thought of by the instructor (who presumably asked you to be his TA, or at least accepted your application). It might also let you get to know the professor teaching the class better, which will be helpful for your application if he/she can write you a letter of recommendation. But beyond that, it won't be a huge bonus in and of itself -- its mostly a signal correlated with other good things.
It may depend on:
- For instance, teaching is compulsory in most American universities while in the UK you are only "encouraged" to teach, with some exceptions.
PhD on what?
- It may not be that important if you are going to do a PhD in theoretical mathematics or a more "industrial topic", while the opposite may happen if the PhD is in Education.
how good does it look? It looks good, but it doesn't represent a huge advantage. In my opinion, as long as you get all the requirements, the most important part of the application are the reference/recommendation letters.