Whenever you can, visit sites you are hosting. Even if your host guarantees 99.9% uptime, it never hurts to check on them every once in a while. Speed is another thing you want to check. See if the sites are slow or fast to load.
Keep in mind that the speed, or lack of it, isn’t solely up to your host. Loading times, as seen from your computer, vary according to your connection speed, your isp, and the loading speed of the hosted site itself (depending on how large the files that need to be loaded are). So, if a site seems slow, don’t rush in to blame your host. Check again at different times of the day. When you finally contact your host, ask them to help you ascertain where the problem is.
If you do note slow loading times on all of the sites hosted by your reseller account, do let the host know that there’s a problem. Sometimes a single account, maybe even on your own reseller program, or on anothe rone on the shared server, can bring down or slow down, websites across that server. Your host needs to know that there’s a problem so that they can tend to it.