This is 100% optional and unless you have some significant bandwidth restriction or data cap on your internet it is likely to be worth the effort.
When you scrape an item Kodi looks for local art and if it doesn’t find it then it takes the info it got from scraping about where the online artwork is and it downloads that. Kodi at this point displays in the UI art that it just downloaded and then as a separate process saves a scaled copy into the “Thumbnails” folder and makes a note of that in texturecache.db. From that point forward Kodi will only bring up the artwork that had been saved to the cache. In a MySQL setup the clients that didn’t do the scraping will automatically generate this cache on demand. The location of what art is to be displayed is stored in the shared db so when the clients go looking in texturecache.db and don’t find an entry they just download and cache on their own.
I don’t see a media manager making a difference. You could try changing the resolution of the cached thumbs and see if that helps. You would need to edit your advancedsettings.xml file and include something like…
<imageres>720</imageres>
<fanartres>1080</fanartres>