[SDPhantom]

To provide my own opinion on frame stratas. As Phanx mentioned. DIALOG and TOOLTIP should only be used sparingly and appropriately. Also mentioned, a tooltip is a window who's sole purpose is to provide additional information about the object you have the mouse hovering over. A dialog is a window that requires immediate attention from the user and usually requests for an immediate reply.

In almost all cases, you should never have to mess with a frame's strata level. If you have a frame created as a child of another frame, it will inherit its parent's strata. If a frame has no parent, then it will gain the MEDIUM strata by default. If you absolutely need to have a frame above all other standard frames, then you may use the HIGH strata. Any higher is not recommended. If you wish, you may use frame:SetToplevel() to make your frame raise above other frames when clicked. Also having an object in a different strata as its container will expose your entire frame to graphics glitches related to overlapping frames.

Unlike application programming, you share the entire addon environment (variables, functions, and other objects) with other addons in addition to the default UI. Following the guidelines Phanx has stated before will ensure your addon plays nicely with any other addons the end user may have running on their system.