I used them with my last pregnancy at 6dpo. I got a line, but it was one of those you knew it was something that came up quickly but really hard to say if it had color.
If seeing a negative upsets you a great deal then I'd at least wait until 11 dpo. I do think at 11 dpo with a FRE, you have a pretty good chance of getting a reliable answer. Of course if you implanted late or whatever, well it might still be negative, but for many that's a good time to use one of those tests but you'd still likely only get a very faint line.