If a sewist puts together a garment without checking for fit prior to cutting, then essentially she is investing a lot of time and money to discover that the garment does not fit. At least when shopping for ready-to-wear we learn of poor fit the minute that we see ourselves in the mirror in the fitting room. Making a garment that fits poorly takes a lot of time!
For people new to sewing the discord between the size they wear in ready-to-wear and the correct size pattern is often the first hurdle. I am sure that there are many people who enthusiastically choose a pattern without comparing their own measurements to that of the pattern. In fact it is likely that the majority of novice sewists don't even know their own measurements.
A reasonable person would feel safe cutting a size 12 pattern if they usually wear a size 12 in ready-to-wear. I actually usually wear a size 8 in purchased clothing, but I use a size 12 pattern. This is not a huge problem unless we don't understand this discord.
One of my favorite fitting books is "The Perfect Fit" from the Singer Reference Library.
The copyright on my copy of this book is 1987, so it is far from current. That however doesn't mean that it isn't a very useful book! The photographs of real women in ill fitting garments accompanied by very clear photographs and written descriptions of flat pattern alterations are the most user friendly of any fitting book I have ever read.
The dated 1980's styles should be overlooked as the information is so valuable. Actually for those of us who sewed in the 1980's it is really sort of fun to revisit those styles!
I have recommended this book for years, and find that it is always available at half.com www.half.com or from Amazon. I have ordered copies for as little as $.75 plus shipping at half.com.