It depends on the context and the reason you are using the accent. If the work is to be performed by actors as a play or screenplay, accents are important for those people to get it "right" (which is to say, the way you, the author, meant it to be).
Accents should be the spice, not the "meat", however, unless you are writing a story in which the focus is on the "otherness" of foreigners, or the acceptance of differences as a principle theme.
On the other hand, I have written works that have then been performed with even (what I considered only moderately) mild accents and the audiences almost all complained that they had difficulty understanding the characters. In truth, a good deal of the difficutly understanding was a lack of familiarity with the idioms and cadence. (American audiences, mind you.) In Britain, as in other places and in other languages, sometimes the accent is also an indication of social status as well as education or ethnic background that can be easily recognized.
So in summary, if the characters speak a lot of dialogue, consider moderating the accented spellings to ease the reader through it, but by all means, if you can accurately re-create an accent without obscuring the meaning of the words, it adds a great deal. Try reading MacBeth some time keeping in mind that all the characters have Scottish accents (of various sorts), and you might find that the words flow together a little more sonorously. On the other hand, a great accent coach in Hollywood teaches accents by spelling plain words correctly, but "tar" is how Southerners say the word most of us recognize as "tire", or the British as "tyre", though the regimental major from colonial India would pronounce it, "tie-ah".
Good luck.
sincerely,
Stafford "Doc" Williamson