It really depends on how you are preparing the salmon. Most salmon dishes are done very lightly, with little seasoning and sauce added to the salmon to bring out the inherent flavor of the salmon instead of over powering it. In which case, most salmon dishes prepared like this do not like to overpower the salmon with side dishes that may be too strong as well.
So if you have a lightly seasoned or sauced up salmon piece then light tasting sides are best. Rice, mashed potatoes, non sauced pasta, green veggies (not the strong ones), and such all make for a very light meal.
Some people like to do a special sauce that is a bit more complex tasting to mix in with the Salmon. A vera cruz sauce would be an example. Or even something a bit more spicy like a habenero and pineapple sauce. When putting together a dish of salmon with a stronger sauce on top then stronger tasting sides are recommended. If the sauce is fruity and sweet tasting then a nice cooling counter balance is usually done. For example I like serving gazpacho with salmon that has been drizzled with my habenero and pineapple sauce. The cooling taste of the cucumber works well with it. Then adding something strong and starchy to the mix after that works too. A good rye bread cna work, but there are other things that can be used.
The point is pick what compliments the main course of the meal when preparing a good meal. If the overall theme is light tasting, then stick with light tasting foods. If going for a stronger punch, make sure to counter balance whatever is used with sides that add to the counter balance. Otherwise the entire meal tastes way too one sided without the counter balance.