Taking into account web browsers, operating systems, iphone, blackberries, etc
It is safe unless you set the proper encoding type of your pages. That's reason why you see various languages of the world in html pages.
If you define properly encoding in html headers, then it is generally safe. However some devices might have problem displaying the character, but it really depends on exact character.
It depends on the character and the font(s). You will need to experiment with the characters you are interested in.
Remember that CSS allows you to apply more than one font to a block of text: if a particular character is not available in your first choice font, but is available in your second choice font, the browser will use the second choice font for that character, even when it can use the first choice font for the rest of the text.
(This does all assume you have set the character encoding correctly.)
While "safe" -- either by correctly setting the encoding when delivering the page or by escaping using &#xxx;
-- not all users will have fonts that can render these characters. Particularly "dingbat" characters (accented Western text characters are generally supported anywhere).
So as long as you're OK with some (relatively small) percentage of your users seeing a box rather than a glyph, go ahead.
Take a look at WGL4. It's list of characters that should be shown correctly on windows machines and I think this means it will be shown correctly on majority of devices.