It's not given to 'speed the placenta up' but to reduce the risk of post-partum haemorrhage.
You make an informed choice depending on your risk of bleeding badly. How many births have you had? How long is your labour? What interventions? How big or small are you? Etc.
Weigh up the risks and decide then.
It can also make your blood pressure go up, your temperature go up, nausea, shaking, before you even start thinking about oxytocin and what's inhibiting that.