Multiple domains under on plan GOOD or BAD for SEO
Add-on domains are one you can consider without worrying about negative seo implications. Although added-on domains exist within sub-directories of the account domain, in all practical terms they are completely self-contained and appear to search engine bots and users alike as no different to individually hosted sites.
I personally used to use the add-on domains feature of Cpanel and never found reason to be concerned over how added-on sites appear to se bots or users.
However, as my domains have grown in number and those domains have in turn become active websites, I felt it was better to 'trade up' to a reseller account as it gave me greater overview as well as greater direct control over every aspect of each domain/site.
(Certain control panel features and facilities aren't individually available to added-on domains.)
Reseller accounts can be bought for reasonably low amounts now and if you have any intentions on managing/overseeing a number of your own or client sites, I would certainly recommend that you track down an entry level reseller account.
Having access to a control panel for each active domain makes managing each domain considerably simpler. It also helps to keep each domain's directories less cluttered and more manageable as you're no longer adding sub-directories for every side-project and client's site, as is generally the case when relying on add-on domains.