If you never write large files to your SSD, except to perhaps install an App, don't worry about it. Lack of capacity impacts the garbage collection's ability to remap free blocks and maintain write performance. Read performance is not impacted in any way by spare capacity, or lack of it.
If you use your SSD's for data storage and you are consistently writing to them, I would recommend you shoot for 20% spare capacity or more. If you get down to 10%, start taking action to clear some space or add a new drive. Otherwise your write performance will suffer, particularly for the period immediately after writing a large file as the garbage collection works overtime to reorganize the free space. But chances are, if you're down to 10%, you're forced to take action anyway as you're running out of space to store stuff, not so much because your performance is impacted.
SSD's have little to nothing in common with HD's so don't let old-school HD thinking impact your SSD strategy.