Well, I don't remember if a recent customer asked me that specifically, but I committed one of the deadly sins when I told her we needed 10 single rolls (5 double roll bolts) to hang her master bath when 12 S/R was what I really needed. Sometimes we paperhangers are more concerned than you realize not to over estimate actual rolls needed. I try to keep cost down, but it's not a bad idea to order a little extra, just in case.
Since I think I know every trick in the book to conserve and stretch paper..if a room takes 12 rolls-It takes 12 rolls!!
Fortunately, I had several rooms left to do and my client and her husband were very forgiving. I worked on that job for several months--in stages--and sadly, they ran out of rooms for me to do.
Footnote: The paper I was short on was from a national wallpaper company that went out of business. I had to call on my "go-to-girl' Dana, at Total Wallcoverings (www.totalwallcoverings.com) to beg, borrow or steal a double roll for me, and she came through when I couldn't even find it nationally!! Lucky me! Thank you Dana.