I only use scent on corn. Like Super says, it can affect the action of lures. Also, when using plastics (i.e. hoochies, beads, etc.) the scent can break them down. If you don't deep clean the lures, eventually the lures will stick together, melt, and be worthless. This is especially true when you put scents INSIDE the hoochie. I also don't like to put scent on dodgers either because it can stick and degrade the plating on them. Again, this can all be avoided if you deep clean your lures/dodger. The best way to prevent breakdown and corrosion caused by scents is to avoid contact. Scent only the corn and you're good to go.
I also don't believe that you can have too much scent. I prefer to leave the biggest scent trail possible to draw in fish. To accomplish this, I will not only scent the corn, but I will also put some scent on my DR ball. I have even seen some people use scent pads or bags attached to DR balls that slowly release scent.
When trying to decide what scent to use, I will do one of two things. I like to change only one thing at a time to see how it affects the bite. 1) use two of the same lure/dodgers with different scents. This tells you what scent the fish are keying in on. 2) use different lures/dodgers with the same scent. This let's you know what lures/dodgers the fish want. I usually start with #2 (different lures/dodgers) to find what combo to use. After I have found what combo works best, I switch to #1 (same lure/dodger with different scents). If you use a systematic approach, it helps you to discover what works best.
For corn, it should go without saying that shoe peg corn is key. That as not been mentioned yet, but I thought I should mention it. I always split up my corn. I'll take one can of corn and put it into 6 or so of those small Tupperware conatiners. Scent some, and leave some plain. This way I have a variety to use at my disposal. Whatever I don't use, I refrigerate for later. It lasts longer than you might think. I have used corn for up to a month before. If the corn has been in there for too long or is questionable, toss it. At less than $2 a can, most of us can afford to use new corn.
As far as what scent to use, the sky is the limit. Last season my go to scent was bloody tuna. Crawfish, anise, krill, garlic and some home made concoctions (mixtures of previously listed scents) were tied for second. Hope that helps answer some of your questions. Good luck.