If you're competing, you should probably know your 1RM. However, you may not have done it recently, so you can't always rely on it. There are different ways to approach this, but I'd say it's pretty common for coaches to consider the opener a lift that you're totally confident you can make, however it relates to your 1RM. Nothing worse than missing your opener.
You can go into a meet intending to lift weights you've never lifted before, assuming your training is effective. But the openers should probably be weights you've done before and are pretty confident with. This may be in the 90-95% range, or it could be even higher if you're really consistent hitting heavy weights. It's different for each lifter.
When you have to hit certain numbers, say to qualify for another meet, opening like that may at times mean that your weight increases are larger than you want, so you may choose to open higher, but you'd better be squared away and hit those openers; there's no point in being close to the total you need with your openers if you can't lift it.
I've personally opened with the numbers I needed for a QT before because I hadn't been training particularly well and I just wanted 3 shots at the weight. Turned out I needed all 3 for the snatch, but got the CJ on the opener. The last meet I did, I opened with the QT numbers I needed as well, but it was because I knew I could get them done easily, get the QT out of the way, then go up according to how I felt.
So the long and short is that it depends on the circumstances.