[Jim Lester]

For Reading systems based on RMSDK (Nook, Bluefire Reader, ADE 1.8 Mac, etc...) the font changing system is based on user stylesheets, which carry a higher level of precedence than a normal stylesheet. Placing the font definition in a body rule (or even just a p/div) rule should be okay. Placing it in a class rule (ie p.myClass or .myClass) will cause problems with overrides for those readers (this is what InDesign does by default, BTW).
Using a class rule won't cause problems with Nook Color/Tablets, and soon not on the NST, but that's a whole different can o' worms.

I don't know how Reading Systems (iBooks, etc...) not based on RMSDK work for this, and would suggest testing to make sure you get the results you desire.

That being said, I agree with Jellby - as a general rule don't embed the font.
If you really think you need to embed the font, please keep your CSS clean (only use the level of specificity you have to).

Note: This is my personal opinion, and not necessarily that of my employer.