Been with HG a few years now from single parent domain evolved to reseller package. I do not resell, just use it for managing my sites. I think what you will enjoy is being able to organize your growth by creating new accounts. No need to wade through 20 or 100 addon or subdomains in file mgr. Just put one to three per account. Huge time saver.
HG hates spikes even with reseller accounts though. Think about timing any crons (autoblogs, autopostings, mailings, etc) and traffic to balance the entire reseller account. Maybe even do like I do and host your auto blogs and directories elsewhere, and keep your money sites at HG for the dependability, and plan your growth as such.
I've run up to 70-80 domains that way, but at the same time was able to crash a server with only a poorly coded crappy article directory script and a dozen or so WP blogs about two years ago.