I shoot in Field Target competition. Is accurate ranging important? Hell yeah. Because in US field target, these devious match directors sometime use teeny tiny 3/8" kill zone at 10 to 13 yards to challenge the shooters.
As post #2 stated there are rules about how close you can get to the line and which part of your body cannot cross the line.
In many international precision shooting competition it's the foot that cannot cross the line.
How far between shooters foward foot to the target on a 10 meter range?
www.airgunnation.com
This is only regarding USA Field Target. The firing line is for two purpose.
1. It is a reference point for the match director to ensure targets are set legally between 10 yards to 55 yards.
2. For safety, we don't want shooters exposing (majority? of their) body parts beyond the firing line.
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Here's the actual answer to you question (I think)
Whether in competition or hunting, does it matter where you range from ? As long as you are ranging consistently. Be it from the tip of the barrel, front of the scope, back of the scope. Choose a reference ranging point and stick with it (and know your pellet drop from that ranging position)
For example, in field target, I get into my shooting position (usually sitting down on a bum bag). Then I range. The only ranging instrument I'm allowed to use is my scope. I range complying with posting #2 (a majority of my body parts behind the firing line and my foot behind the firing line). Say the match director was really devious and set the 3/8" target at 13.8 yards from the firing line.
Am I going to range that target as 13.8 yards? No, for me it's always going to be longer than 13.8 yards.
Say I range it at 14.2 yards, I consult my pellet drop sheet and I shoot it at 14.2 yards. Officially, I've just shot a 13.8 yard target. Unofficially, because of my shooting position and my reference ranging point (to me), you could argue that I just shot a 14.2 yards target. 13.8 yards matters only to the record keepers and complying with the rules. 14.2 yards matters to you as a shooter (given the way you range, your pellet drop, how consistent you are positionally and optically ranging a target, assuming there are no issues with your rifle and scope )
So bottom line. Chose an arbitrary ranging reference point. Always use your ranging equipment in a consistent position. And know your pellet drop in relation to the "unique" way you range.