I plan to retake the GMAT, but I'm already happy with my IR and/or AWA score. Can I skip next time?

This is an interesting issue to talk about! After reading here, you might also want to check out this GRE blog post on whether you should skip the AWA. That will give you Chris' take on the issue, and even though the discussion is about another test, the basic ideas are the same.

Whether or not you can skip a given section depends entirely on how the schools you wish to attend deal with AWA and IR scores. You should never skip a section of the test before you know with 100% certainty how the admissions committee(s) are going to view your scores. In broad strokes, there are three main ways that universities handle the AWA and/or IR scores:

1. The AWA/IR is completely ignored/not used for assessing a candidate.
2. The AWA/IR is used as a benchmark score (e.g. you need to get a 3 or higher, but simply passing this cutoff is all that counts).
3. The AWA/IR is actually used as a vital component of assessing your readiness as a candidate.

Another key thing to consider is that the IR is relatively new, but as of June 2017, all valid GMAT scores include the IR. Schools that have not looked closely at the IR in the past (because some students with valid scores didn't have the IR portion) may now decide to do so. Be careful making assumptions about whether the IR counts!

In short, the AWA/IR is not always a determining factor of a student's admissibility. While it can be an important part of your score, a lot of people tend to focus on just quant and verbal as their "main" score. Do not assume that you can mix and match scores from multiple tests. While it may be possible, it is not typical. 

To summarize, be very aware of what the admissions committees will want. If you do not know, assume that each test session counts in its entirety, and do your best on every section.