Visportals are meant to block visibility where the player is not meant to see (e.g. behind a door).
This helps the engine to decide not to render, or (for all intents and purposes, because there are more nuances) even think about, anything that lies behind the visportal until the door opens.
You place visportals in order to ensure performance for your maps.
Because your map goes from room to room you don't have to worry about doors and blocking things with visportals, so no worries there.
Placing visportals can be interesting though if your rooms are very detailed and have L shaped corridors or parts that can logically be blocked off by a visportal.
This explains visportals in a more detailed way: http://www.iddevnet.com/doom3/visportals.php
Unless your rooms are L shaped, have many corridors or are very detailed there's not much to worry about.
You can always check to see how much the engine is rendering by using the below console commands which will show all tris that are being rendered at that moment.
The r_useScissor 0 option is very important. As even though the areas aren't being rendered by the videocard, they are still being sent to the video card by the engine which is a very slow process.
So always test your visportals with r_useScissor set to 0. (Of course don't forget to set it back to 1 later on when you playtest your game since when you turn scissors off the data will not only be sent to your videocard it will also be rendered!).,