I usually break a ball or two from center table on a table I’m unfamiliar with just to see how it goes for 8/9/10 ball. For the most part I really only break from the rail playing 9 ball, which will be a ball or two out from the rail. Other than that I’ll only break from the rail in 8 ball if I’m going for a second ball hit in hopes of having an 8ball break for the win.
Every table is different though, some break(rack) better than others, the humidity will affect how a table breaks(and plays), but that and the condition/quality of the balls have a significant impact on the spread as well. The squareness and/or tilt of the rack will also make a big difference in whether or not balls are dropping.
I’ve rambled a bit out of line in regards to your question but these factors determine as to how far off the rail/how far from the headstring I’ll break from.
If you’re playing competitively you should always check the rack to see if it’s good(enough), but more importantly, check if it’s consistent. So when you find a spot that works you can continue to break from there, and vice versa. It’s hard to learn from a break if you’re getting a different rack that’s completely different each time.