FACT: every time someone unsubscribes from your email program a fairy dies.
OK, so that's not true but it might as well be. Why? Because by and large the cost of an unsubscribe is inflated by emotion and that's led to a best practice that could be costing you millions.
The key to A/B split testing in email is patience. Wait as long as you can to analyze results and you're more likely to get a true picture of what will happen when you roll out your eventual email campaign.