Question: Space isn't empty - there are tons of things floating around in it. Seem of these things are pockets/molecule of gas - and a space probe is very likely to encounter some of these at some point in its lifespan.

Are there high enough concentrations of gasses to justify "streamlining" spacecraft? Are there any examples?

Answer: 
Let's just assume the interstellar density of about 10^6 particles/m^3. Let's also assume they are hydrogen. Plugging those numbers in to the [drag formula](http://en.wikipedia.org/wiki/Drag_equation) $F\_D\, =\, \tfrac12\, \rho\, v^2\, C\_D\, A$, and let's assume that 1mN is the first force that is significant enough to merit correcting for. Putting all of that together, assuming a hideous coefficient of drag of 2, and an area of 100 m^2 (For solar panels), the appropriate velocity becomes 2,496,000, or almost 1% of light speed. 

Bottom line, Aerodynamics only makes a difference if you are going really fast, much faster than anything we've sent so far.
Short answer is NO.

Within interstellar space or within our local solar system, aerodynamics isn't going to make much of a difference. And from @PearsonArtPhoto's calculations, at the speed at which they DO make a difference, the particles are going to ram into the probe with enough energy to erode the outer-surface. Gases and particles in interstellar space are often ionized (i.e. charged) and some atoms are very hot. It is akin to driving a car at 100mph through a dust-cloud - your windshield is going to be poc-marked and if one drove long enough, it might crack and fail. 

With that in mind, you'd want to make the cross-section of a ship small to deal with erosion more than aerodynamics. Probably made of thick and durable material, and ideally shielded somehow - possibly with a magnetic field to help push particles away from the ship.

Have a look at: <http://en.wikipedia.org/wiki/Interstellar_medium> for a bit more detail.

For a real-world example, Voyager 1 - traveling at a brisk 17 Kilometers per SECOND away from the sun isn't very aerodynamic at all. Big clunky right angles, a giant radio dish on the back, etc.