Depends how much boost you want to run. What you have to bear in mind is if your running 1.5 bar and you lift off, the turbo is still going to produce some boost before it stalls.
This means your intercooler pipe work will be seeing more like 2.0 bar when you lift off. This sudden surge in pressue with no where to go can cause pipes to pop off and in more serious cases like Colin's split the intercooler.
Its your choice but a dump valve can prolong the life of the turbo, stop the turbo from stalling in between gears for quicker spool and stop damage from pressure spikes. You also dont have to have one that makes a noise. The HKS racing dump valve is almost silent