Answers in no particular order.
First, if you feed all sources (both video and audio) through the receiver, with only one output to the TV, you will have to use the receiver at all times. You can ignore the rest of this post
If you feed the video to the TV and the audio to the receiver, thereby using the TV to select the video source, you can most likely get by just using the source and the TV without needing the receiver.
To get 5.1 sound from your DVD player or STB, you will need to feed either a toslink (glass interconnect) or a digital coaxial cable (usually an orange jack) to the receiver. Those red/white jacks on the source should be fed to the corrosponding input on the TV to allow them to be used w/o the receiver being on. The sound will come out of the TV's speakers. The same applies to the cable/DVR STB.
In both above cases, when you want to use the receiver and big 5.1 system, simply turn the TV sound all the way down and control the sound via the receiver.
But, a lot depends on what inputs your TV has. You don't offer too much information on that and it's pretty important if you want more specific answers.