this is true....
but to answer your question....here is a true story
General Motors put out a bulletin a while back for 01-06 models. I was working on a 2500HD w/ a Duramax engine that had a hole in the piston and cylinder damage. The engine had to be replaced due to the damage. We are required to call into GM for approval prior to any engine assembly be replaced. When I described the damage, they could tell that the damage was directly a result from an aftermarket add on power programmer. They declined the repairs under warranty and the customer forked out over $11,000 for a new engine.
Here is a small part of the bulletin that deals with this....
I am not saying that the programmers VOID the warranty all together, but if you have repairs that are needed as a result of damage caused by the prgrammer- GM will not warranty it. You just have to be carefull when adding one.
I myself have a programmer on my truck. No you don't need one, but the extra power is nice. I have an EGT gauge (pyrometer) to moniter exhaust temperatures- very important when adding a tuner to a diesel.
It is true that when you bring it in for service, you should take the programmer off. This way if they have to re-flash (reprogram/ update) the PCM you won't lose your programming.