I recently designed an email using fractional pixels for the text size. and letter spacing.
EX:

<td style="font-weight: 300 !important; font-size: 16.5px; letter-spacing: 0.2px;">

I was told that fractional pixels for font size and spacing can cause errors, but when rendering and testing on major clients, I'm not running into any problems. Does anyone have best practices when using fractional pixels for these elements?