In addition to the ROMs and POSE, the last thing you might want to obtain is a "skin." For the most part, a skin simply provides POSE with the ability to take on the appearance of a specific target device, such as a Palm m505 or a Sony Clie 615. POSE does come with a generic skin, so technically you do not need to obtain extra skins.
Although the use of skins can make for a more eye-catching POSE experience and will almost certainly impress all your friends, generally speaking a skin has no positive or negative effect on the actual operation of POSE. The one exception that I will point out is in the instance of a device with a radically different form factor. For example, the Handspring Treo communicator, which is a combination cell phone, PDA, and wireless device, comes with an integrated keyboard as well as a fairly drastically remapped set of buttons on the device. It can be somewhat disorienting to emulate such a device using the generic skin, and some functions may only be accessible by using the skin of the target device. But at present this is the exception rather than the rule, and personally I tend to use the generic skin more often than not.
One final note on skins, ROMs, and POSE. Several Palm OS licensees ship a custom version of POSE that is specially enabled with options and extensions that are unique to their device. If a device manufacturer offers it, it is generally a good idea to use the device-specific version of POSE with the appropriate ROMs.