How many US institutions use letter-grades without + or - indications for GPA? I'm currently reviewing graduate applications and had discussions with friends at Carnegie Mellon University. There, undergrad grades are assigned as A, B, C, with no + or - designations. Thus, a student who might get an "A-" (3.67 towards GPA) would always be rounded up to an "A" (4.0) and these friends said that "B+" (3.33) would almost always secure an "A" as well.

In this grading scheme, there's a huge range of performance that might earn a 4.0 GPA. A student performing B+ / A- work (e.g., 3.5 GPA at my school) would be rounded up to a 4.0. That makes a big difference when I compare to other schools that use the traditional A-/B+ US grading scheme.

As far as I know, this is a rare grading scheme - but I'm curious if there are other US institutions that use this scheme?


Source: CMU University Grading Standards
There is quite a lot of variation. Some don't even use 4.0 as the top. In some places the grade assigned is a letter grade, not a number. The numeric conversion, then, doesn't involve rounding. It is just the number equivalent to the letter. The numbers are used to compute overall GPA, but do not relate to numeric grades in and individual course. Some courses (mine) have numbers between 0 and 1000, with letter grades defined by a table, but the university only sees the letter grade. 

Other places the grade assigned might be a number such as 3.67, though I think this is fairly rare in the US. 

Plus and minus grades is a fairly recent refinement (50 years??). One university  in my past had problems when they started to give A+ grades since graduate schools then considered the place to have a 4.5 scale, not 4.0. This effectively downgraded gradates with "perfect A grades". The had to rethink the system in a hurry. 

But to find out which for some set of universities you need to do the research. Contact the registrar's office if you want the true picture. 

And some parts of your question suggest that you are confusing an individual course grade with an average over several courses, each weighted differently. The weights come from the "credit hours" assigned to the course. Thus a course that meets five hours a week might be "worth" 5 credit hours and an A there contributes 20 "quality points" toward the GPA. A course meeting one hour a week would "count" less toward the GPA. 

I took a writing course once in which the prof defined his grades thus:

An "A" on a writing assignment means that it is written as well as it can be written. A "B" was assigned if it was written as well as any college student could expected to write it. Etc. I forget the rest. The course was 3 credits so an A gave you 12 points toward your GPA, since the conversion was the typical A = 4.0. 

Add up the "quality points" and divide by the number of credit hours completed to get the GPA. But there was no "average" in a given course. Just a grade. 

If I had to guess, I'd suggest that more places use plus and minus grades now than don't. Before ubiquitous use of computers it was harder to have a more nuanced system, but now, the prof just needs to check a box for each student and the computer takes it from there. And some older, more conservative university systems want to stick with tradition. Why change what ain't broke? So, if you got your degree from CMU in 1960 and your grandkid did in 2015, your grades are on the same scale. Nominally, at least.