Unless you chart your BBT, you don't know when you ovulated so you don't know when your period is actually due. Best to wait 1 day after your longest cycle.
From the newbie link:
HPT accuracy DPO
10 dpo : 35%
11 dpo : 51%
12 dpo : 62%
13 dpo : 68%
14 dpo : 74%
15 dpo : 80%
16 dpo : 88%
17 dpo : 92%
18 dpo : 99%
DON’T TEST EARLY!!!!