How much do midterm grades matter? Would a few B+ or A-'s hurt a student?
They matter. Whether you get accepted, rejected, or waitlisted will not be the result of a few B+'s or A-'s.
While I agree that a few A-'s would have no bearing on whether a student is accepted, rejected or waitlisted, Harvard expects students in their senior year to be on an even-keel or be having an upward trend. A few B+'s in an applicant’s senior year would seem to indicate that an applicant was either struggling with the material, or had senioritis – neither of which bodes well for the applicant.
You do realize that in many schools, an A- is a 90 and a B+ is an 89?
I think you are unnecessarily worrying the OP.
^^ At many high schools, letter grades cover a range: an A- is a 90 to a 93, while a B+ is an 87 to 89. You are correct that there is little difference between an 89 and 90, but there is a world of difference between an 87 and a 93!
At some colleges, when the Admissions office recalculates a student’s GPA, they throw out the pluses and minuses altogether. So a B+ becomes a B, and A- becomes an A. Unfortunately, most Admissions Offices, including Harvard, do not specify if and how they recalculate a student’s GPA.
@rhandco @gibby @skieurope I have 2 A- (the rest are As)…but of course its Harvard 
so I’m nervous, especially in the midst of all the other great applicants…thank you all for your answers
People who are freaking out about B’s need to chill. It totally depends on your school. At my school, getting a B in a challenging course is normal. Plenty of kids in the top 10% have B’s. A B is not considered “struggling”. A B means you were in a difficult class, and it kicked your butt a little, but you pulled it off. What’s the point of taking a challenging course if it isn’t going to actually be challenging? Of course at other schools, it may be easier to get all A’s in every course, and anything below a 4.0 is not good enough. Grades aren’t a black and white thing, especially when it comes to different schools. On the bright side, your regional admissions officer is familiar enough with your school to know how rigorous your courses are, and what is considered above average in your classes.
^^ That’s not the advice that I would give, especially for tippy-top schools like Harvard. For example: http://talk.collegeconfidential.com/harvard-university/1730604-advice-for-a-deferred-applicant.html#latest
I agree with @gibby. Those that are deferred should maintain the highest grades possible and should not be dropping any classes…would look bad. Moreover, for those that were accepted early to H who have also applied to S, Y, P, M should maintain the highest grades possible…
…however, those who are one and done (did not apply anywhere else in the regular round)…can relax a little without allowing full-blown senioritis to set in…
…and this information can be applied to those who were deferred or accepted early to the above schools…
Wow I’m amazed that the GC of that student called Harvard AO. I wanted my school GC to write a support letter to Harvard after deferral. The look I got from her was not pleasant.
How many Harvard acceptances does your school get per year? We might get 1-3 at most into the most competitives, so the Counseling department would be on it in a flash.
@JustOneDad, the answer is zero. And one if the question is how many applied since my high school established.