No, that expression is not a good one for that situation. “Not get boxed in” means “make sure that you have choices/options available.” Example: ‘I don’t want to buy an expensive house because that will box me in to staying in a high-paying job that I don’t like, and I won’t have the freedom to choose a lower-paying job that I like better.” For your situation, “better safe than sorry” would be a good expression. “I decided to overprepare for the test to make sure that I get a good grade. Better safe than sorry!”

I agree with John’s comments about getting boxed in. As far as “better safe than sorry”, that would be ok if it was somehow “unsafe” to fail. When going into the mountains, you should always bring a coat, even when it is warm out. The weather can change unpredictably and it is better to be safe than sorry. Other possibilities. I don’t want even the slightest chance of failure. I don’t want to cut it close. (Barely succeed) I need an 80 to pass but I’m shooting for a 95. I want to have a lot of margin.