Depends on the version of the standard that you are following. The current standard uses the term "may" so it really makes no difference. The '94 standard however is more directive. ¶3.3.2 states that "the datum feature symbol is applied to the concerned feature surface outline, extension line, dimension line, or feature control frame as follows:... (f) placed above or below and attached to the feature control frame when the feature (or group of features)controlled is the datum axis or datum center plane."
While it could be argued that the direction is a bit vague, I have known checkers who interpret the standard more strictly than others and I usually don't feel that it's worth battling over.
"Know the rules well, so you can break them effectively."
-Dalai Lama XIV
