Personally, I would tend to follow print. You should be aware though that in Nemeth an arrow can also be shown with the shape indicator using braille cells. Since you have posted this question under tactile graphics, I assume that you are talking about an arrow that appears in a print diagram. Sometimes we describe a print diagram in a transcriber's note that is used in combination with a basic (simplified) tactile graphic. Can you attach an example .jpeg or .pdf scan of an example where you consider that the arrow only has "visual purpose"? I would hate to make the assumption that an arrow has no had no significance, and could therefore be omitted, unless I was familiar with the subject matter, and there was no doubt from the surrounding text that this was true.
I'll check back again to see if you find an example to post. Thanks.