urh... it's kinda situational innit?
I mean, if he's got no critters out, and you really need the land, then sac it.
On the other hand, if he's gotta 3/3 or something, you need to block.
If you have something in your hand that costs just one mana more, then why keep the farmhand?
Of course, if he's got no critters and you have enough land, then you can use him to attack.
Important things to remember:
Farmers get land anytime, even at the end of your opponents turn.
Farmers get you tapped land, useless on the turn their used.
Human baby's taste like chicken, but lose that juicy flavor with age.