Wooden butter knife sanding?

It's hard to say exactly without knowing what wood it's made out of and how it was constructed - there are many factors to the durability of wooden tools and utensils. If all you are using this for is spreading butter, margarine and the like then you shouldn't need to do too much care, just wash it and especially dry it thoroughly after use. It's rotting that is usually the issue with wooden utensils.

I wouldn't sand it unless bits of wood start sticking out of it, which could happen depending on the direction of the grain. Sanding without having a good reason could actually cause you more problems then it would solve as you'll be rubbing off any finish it came with.

I believe that in order to use your wooden knife without worries, you just need to regularly clean it with hot water, a bit of dish soap and some scrubbing - same as with wooden chopping boards. If you want to maintain a nice sheen you can use a little bit of vegetable oil that you rub into the wood using some kitchen towels.

I don't see how sanding would affect the "food worthiness" of a tool, or why it needs to be repeatedly treated in this manner. What are you expecting the sanding to accomplish?

If you are asking if it's safe to eat with, let me point out that it's the same material as cutting boards and wooden spoons. Wood wicks away moisture quickly so has natural anti-microbial properties. You just need to make sure to let it dry completely, and in professional situations they can be cleaned with salt.

First off: wash by hand, not a dishwasher (kicking in an open door). Heat and moisture make wood warp, so the less exposure to either/both the better. You should not sand it unless it gets rough (raised grain) chips or any other damage.

The easiest and best way to make sure it is and remains food safe is to apply a food safe oil. Do not use olive oil as it can go rancid and stink. Lemon oil, walnut oil or linseed oil are good choices. There is a saying a saying that to fully impregnated wood with oil you need to soak it in oil, each day for a week; each week for a month; and each month for a year.

I would just wash it gently after use and apply your oil of choice every 10 times or so. Just a little bit applied with a paper towel.

In regards to sanding, that removes some wood, exposing "blank" wood, so apply oil again. I assume the wood itself isn't poisonous, so that should be ok.