is that sentance from a quote from a navigator? or actual material? My interpretation is any navigator would say they are the only way through the warp without damnation, for the sheer fact that that's their entire role in society. Hence why they spend so much time and resources to find and destroy any other form of travel without navigator help.
I would go with this. The fact is that for anything other than very short jumps in very stable routes which have been in operation for centuries the prospect of going without a Navigator should be terrifying to your avergae citizen. This is in part becasue of the nature of the Warp but also a result of a campaign of misinformation and imposed ignorance (which, basically, every aspect of the Imperium thrives on…) which is the base of the Navis power.
The Captain may decide he wants to risk those jumps, but consider the impact on the morale of the crew. Even if they survive a jump or two, they would likely panic. SOme might refuse to co-operate (imagine them barricading the warp engine rooms and refusing to complay with orders, etc.) This is, of course, unless there is no other option left (Navigator dead, say).
So, as others have suggested: if it's in-system shipping, or very close systems and stable routes, then no Navigator is 'necessary.' If there's even the slightest deviation from the norm or the paths are unastable, then a Navigator should be present. There's a reason the Navigator and Magisterial Houses are so stinking rich and powerful: they can pretty much name their price. And of course, this is where the Shrouded and Renegade Houses fit in: the black market of the Navigation world. I would imagine many of the more shady Chartist captains - who might not employ a Navigator full-time - would still 'know a guy' who can get them a 'Navigator for Hire' if they ever need to make an 'unscheduled delivery' or something.