There are two schools of thought when purchasing appliances.
1. Buy what you like: Typically in the super premium category, you would find a SubZero, Viking or Wolf range, Miele, Bosch or KitchenAid dishwasher and a Best or XO vent.
2. Buy the same brand: You may compromise slightly. However, in the new economy, most companies are offering package rebates which could be up to 20% (Bosch will have 30% in rebates from October 2-12).
There is another reason...Have a look at a few branded kitchens:
When you buy within the same brand, the look is homogeneous.
You can do both. First step is always to budget properly. Second step is to ascertain order of importance. For many people, it is cooking, and for others it is refrigeration. With budget considerations, you would spend less on a dishwasher, microwave and laundry.