Please don't fall victim to the myth that 2L grades don't matter, because they do. Especially if you want to go into big law.
1L grades are undoubtedly the most important but employers do look at the 2L grades. Even if you have the job lined up with them after your 2L summer some firms have been known to spy in on their incoming associates. Plus, just as a general rule, you always want to leave yourself options. Should you be one of the lucky few who have a job at big law lined up for your 2L summer that was attained based on your 1L grades, it is very possible that you may not like the firm afterall once you get there and interact with the people everyday.
Unfortunately, the nature of the beast with law firms is that even if you're a bright individual who does good work, there is no guarantee that you will receive an offer because firms put A LOT of stock into your chemistry with the firm. I personally know of a few law students who are at the top of their respective classes who did not receive offers simply because they didn't vibe well with their firms.
So fight the urge to slack. Save that for 3L (if ever).