Thinking about it, there may be more to wanting no header leaks than just noise reduction..
It could be said the entire purpose of changing over to headers is to in the end create a higher vacuum depression in the cylinder through more efficient exhaust evacuation. Efficient enough to create a higher vacuum in the cylinder, producing better cylinder filling, and more HP.
Unless you're replacing OE headers that are particularly restrictive, much of that vacuum increase comes from increases in scavenging are happening well downstream of the header to cylinder head flange. Probably kind of an accumulative effect that once back at the cylinder yields its' benefits.
If there are leaks at the slip fit, or worse - at the header flange to cylinder head connection, most if not all of any increased scavenging/vacuum boost your headers stood a chance of giving the cylinder will disappear in an instant through the leaks instead of creating more vacuum in the cylinder..
The cylinder won't know there's a header attached if it doesn't get the message.
I do know it doesn't take much of a leak to lower or eliminate vacuum strength altogether. You've got to contain it well if you want to keep it.
