It has nothing to do with being safe. Of course if you haven't got an alignment in a while then do it but even if you are going from 16's to 17's or vice versa the overall diameter of the tire should be roughly the same (if you are smart) so you aren't even lowering or lifting the car much by switching wheels. At least not enough to influence how the suspension holds itself.
FWIW, changes in offset and wheel width can impact alignment settings. If the changes are modest I wouldn't worry a whole lot about it. And I agree with the above statement, if you haven't had an alignment in a while I'd certainly consider one to protect the rest of your investment (I'm assuming you're adding new tires as well).
nice....the picture you have is actually a "sports bronze" slipstream...the bronze is a bit lighter in color....I have them..you will be happy with them...the allignment isent as important as making sure you balance the wheels and tires once there mounted...allignment wouldent hurt though...like Geezer said...protect your investment!