Hi, if i am mixing and not recording should i still have my interface (tascam us-122) connected for cubase le5 to run at its optimum performance?
IMO: Yes – I always kept my Tascam US-144 connected all the time using Cubase Le4.
Are you using a PC - Mac - Laptop 
I’m sure opinions will vary on this one. 
Jack 
The selected soundcard driver will affect the responsiveness of the system due to buffering, so if you want to use say a laptops internal speakers then selecting one of the generic drivers could lead to a certain sluggishness.
Downloading the ASIO4ALL driver would help a lot in this case.
Of course the soundcards buffer setting also affect how much strain is put on the ASIO system, which in turn affects how much load you can have before the sound starts to breakup.
So the answer is not a simple one, and depends a lot on a good ASIO driver.