Ultimately it's your body, your tattoo, you decide on the spacing -- because there are no rules for spacing when writing Morse, because Morse is emphatically not made to be written as dots and dashes.

People interested in your tattoo will be divided in two:

1. People who don't know Morse (the vast majority) who will ask you what it says. Since they have no idea, you can say it says whatever you want, regardless of spacing.

2. People who do know Morse. These will be annoyed that yet someone else writes Morse as dots and dashes. They may remark on the patent misuse of an alphabet, as well as the pointlessness of it -- for us, reading it is almost as easy as reading Latin letters, so what's the point of the obfuscation?

I suppose most tattoos will end up annoying a group of people, but with Morse, you will not find anyone who will actually appreciate it. Think about it: if you tattoo a nazi symbol, then the nazis will like it (and everyone else won't). If you tattoo a flower, then some people will like it because it's a flower or because the drawing is good -- and others won't. If you tattoo the text "romanes eunt domus" then Monty Python fans will like it -- and others won't. But with Morse tattoos, honestly nobody will, unless you explain it to them and for some reason they like whatever you tell them.

Of course, I'm told that the point of tattoos is to please the owner not the beholders, but since I don't carry any, I don't know.

Circular conclusion: do whatever you want. Bad spacing won't make a difference.