If you decided to use one of the natural stones for your project (i.e. marble, granite or quartzite), the answer is always YES! Each slab of natural material is different (after all, it was created by nature) and seeing a small sample may not be an accurate reflection of the slab that will be used in your home.
Even photos may not show the true colors or possibly hide flaws of the slab (i.e. cracks) therefore this step is necessary, and it will save you from a headache down the road.
On the other hand, if you opted to work with a manmade material, looking at a sample should be a very accurate reflection of the actual slab.
The only exception may be material with lots of movement, or heavy veining, as looking at a sample gives you only a glimpse of the entire surface.
Tip: If a full slab is not readily available for you to see, look for photos of full slabs, or better yet, search for projects completed with the stone you are interested in.
