A Bypass valve returns the boosted air back into the intake track.  it typically doesn't have that PHA noise when you let off the gas.  A blow off valve does the samething but vents to atmosphere and gives you that noise.  The main purpose is to relieve pressure build up in the intake tract when you are on the boost and then shut off the thrttle closing the throttle blades.   With low boost do you need it?  Good question.  I think the better question would be that if you are making boost and shut the throttle where does  the pressure go????  it has to go back agains the flow of the turbo.  doesn't do it any good.  My vote would be at least a BOV just to protect the system.

Also consider if you are having boost controller issues and the boost spikes and you let off the throttle.  Suddenly you are in high boost conditions but without a BOV or BPV you will have no protection for your intake system.

I am running the stock 87-88 TC VAM. So the BPV is the better idea. Where in the system would the BPV mount? What about mounting for the BOV? Which is more more complicated system to work with?