For the first year, infants should have no more than 0.4g of sodium (1g of salt) per day. Odds are they will get most of that through naturally occurring salts. More than this can be dangerous for your baby's kidneys.
After one year old, you can start adding small amounts of salt to foods, though it's probably not necessary in most cases. Especially if you find them eating foods like french fries from time to time, that's going to give them a day's worth of salt right there.
As far as sugar, there's not as much risk as there is from salt, though if you head over to this question/answer, you can read an excellent summary of the potential risks. More importantly to me, if you feed your child mostly sweet things, they will eat only sweet things.
This is true even later in life; sugar changes the information your tastebuds give you, and if you go off sugar for a while, you'll start noticing you like things you didn't before. Train your children to like savory foods from an early age, and you won't have a challenge with them eating when they're older.
That said, you should talk to your pediatrician about this. If they instructed you to put some sugar in the cereal, they may have done so for a reason - so find out why. If it's just to make it taste better, but there's no health reason (your baby is not underweight), they likely will tell you it's fine to leave it off. But verify that first, in case they do have concerns about the weight of your baby.
A good alternative to sugar is to give your baby a few vegetables before cereal; there's no need to feed cereal first, and vegetables are excellent sources of interesting tastes. Then when cereal is introduced, it can be mixed with vegetables that the baby has already tried. Make sure you're only introducing one or two foods per week at first, and one at a time (to be able to isolate allergic responses).