Hi Hannah, Thanks for your question! GPA is just one of many, many factors we look at when reviewing a resume. I'm interested in seeing a picture of who the applicant is overall, which includes academic achievement, but also work or internship experience, extracurriculars, volunteering, personal interests, etc.
GPA is rarely my most important factor, but your GPA will tell a bit of a story about you. A 4.0 GPA is undeniably phenomenal, and it shows to anyone looking at your resume that you are a hard worker and very bright. I think the same would be immediately thought of for someone with a GPA of 3.7 and higher. Below that, especially with a 3.5 or 3.6, you'd be hard pressed to find a company that would exclude you based on GPA alone, but your high intelligence and work ethic might not be immediately assumed based on your GPA (the way it would with a 4.0). Other factors - internships, extracurriculars, etc. - would start to carry more weight.
Also, the value of the GPA depends a lot on the type of job you're hoping to get. In highly competitive roles with many exceptional candidates, GPA might be more important. There are some roles or teams where the GPA doesn't matter much at all. Everything above reflects my own experience when looking at resumes.
But it sounds like you are doing a fantastic job in school and you should not be too disappointed if you "get a B somewhere down the road." As I said, the GPA is only one small part of the story of who you are. Therefore, if the trade-off you're considering is between maintaining a 4.0 but at the expense of doing any extracurriculars/interships, vs. dropping to a 3.5 to give yourself room for significant extracurriculars/ interships, I would suggest you go for the latter.
Best of luck and congrats on your successes so far!