You could probably minimize or eliminate erosion on this mild slope with some wood chip mulch. It's a good idea not to raise grade in front of a fence, if possible.

Thanks!

Would certainly be easier if I don't have to try and level it out.

One more thing occurs to me, if you NEED some levelling. If that's the case, since you're only trying to combat potential erosion, you could use individual boards on edge that are attached to stakes securely driven into the ground in order to level up sections of the garden. Let's say, for example, that there is an 18" grade differential from one end of the garden to the other. Instead of creating an 18" retaining wall/device at the low end of the garden, divide the levelling into 3, 6" level changes. Plants can hide any unattractive structures since those would be low height. I would use the easiest, cheapest solution that does the job.