From an old shorthand book (not Gregg) on internet archive had this advice. I changed a few words since they use different terms :
The briefer the forms you use the more illegible will be your writing; for the less of a word you write the more of it you must remember, if you were to read it.
When your memory becomes dim or overburdened by the number of arbitrarily abbreviated brief forms or abbreviations you try to use, you will have difficulty in recalling them; your ability to read will depend upon your fleeting and uncertain memory of the multitude of little fragmentary characters you have made, and your skill in constructing the proper sentence from them.
The more of a word you write, the surer you are to read it certainly and promptly at any time.
I think it would be fine to add abbreviations you would use often, but you do want to be careful about trying to abbreviate everything. Not only for your memory, but also if you have to read it back. Shorthand is short to begin with and the limited writing you have to do makes it fast, so speed should not be the issue here. If you are trying for speed, there are a lot of tips you can use to build up speed.
Also, if you're learning Anniversary, you have enough brief forms to memorize you don't need to add more.
"Gregg Shorthand had achieved the most extraordinary success ever attained by any system in the history of shorthand." John Robert Gregg, 1922