Whether you shot 70 or 120, if you played an acceptable round to post to GHIN, you need to post it. It's the only way to ensure the World Handicap System works as intended. Leaving off your best scores to keep your Handicap Index® artificially inflated gives you an unfair advantage in getting strokes you don’t deserve based on your actual ability. Plus, it makes you a less desirable person to play with from an equitable standpoint. Cut out the embarrassing high scores and it might make your Index look better, but you're only hurting yourself when it comes time to use it in a competitive setting.
There are two major reasons why it's critical to post your round as soon as you wrap up (or as soon as you get home if you can't post from the course). The first is that your score will be one of many data points used to determine whether a Playing Conditions Calculation (PCC) adjustment needs to be made for the course you played that day. This calculation is used to determine whether the course played abnormally easier or more difficult on any given day and whether the differentials of those that played that day need to be adjusted accordingly. The usual culprit in warranting a PCC adjustment is weather, such as rain softening up the greens or wind wreaking havoc on drives. But since this is simply a statistical measurement of expected vs. actual scores, it could also mean the grounds crew just decided to be cruel that day and select hole locations like it was Sunday at the U.S. Open.
The other reason it's important to post right away is that your Handicap Index updates daily at midnight. Posting the same day as your round ensures that your Handicap Index stays up to date, especially if you have another round coming up.
When I mentioned posting acceptable scores earlier, that doesn't just apply to 18-hole rounds. If you play at least 9 holes on a course with Course and Slope Ratings, you can post the score to GHIN. That applies whether you stop after 9 or make the turn but aren't able to finish all 18 holes. Here's how it works:
9 holes - Post either your total or hole-by-hole score, and GHIN will calculate an expected score based on your Handicap Index for the total of the remaining 9 holes to determine your 18-hole Score Differential.
10-17 holes - GHIN will similarly produce an expected Score Differential based on the unplayed holes, with the difference being that you'll need to post a hole-by-hole in order for GHIN to determine which holes need an expected score.
18 holes - Post either the total or hole-by-hole score
GHIN has each of these listed as separate options when posting your score, so you'll simply need to select the correct option and enter your score accordingly.
When reviewing your score history in GHIN, you'll see a list of all the factors that determine your Score Differential, which is eventually used to determine your Handicap Index. Here's an example of what that looks like:
The first column represents the different types of score you can have. Here's a breakdown of the possible designations:
H - A course you have designated as your "home" course. This could be the course where your membership is tied to or if your club doesn't have a home course, the one you play the most frequently.
A - A regular round played anywhere but your home course.
C - A competition score is a score made in a competition organized and conducted by a Competition Committee.
N - A 9-hole round
E - An exceptional score, defined as a score that is 7.0 strokes or lower than your Handicap Index. This will result in an automatic reduction of your Handicap Index.
Your Adjusted Gross Score is your raw score adjusted for the max score on any hole of net double bogey.
Course and Slope Rating are calculations made by your local golf association to determine the difficulty of the course and set of tees you played.
PCC was mentioned earlier here. This value can range from -1.0 (course played easier than usual) to +3.0 (course played harder than usual).
Your Score Differential measures how well you played relative to the difficulty of the course and set of tees. The lower the differential, the better you played. Your 8 lowest Score Differentials of your previous 20 rounds are averaged to create your Handicap Index.