is leaving the computer side panel off good?

For some things, yes I suppose it would be "good" to leave your side panel off. A situation would be if you are having temperature problems and are using an outside fan or something to diagnose and solve a problem. Another would be if you were perhaps overclocking and trying to see your personal best benchmark or something to that extent. Other than that, the extra dust build up that having the side panel off and the obvious risks of something falling inside and harming components present might not be worth the few degrees you would gain. But I guess that would be a personal decision and would warrant a safe area preferably off the ground to keep your side panel off.

thanks u very much

is a 11 inch tall tower considered a mini tower?

i used to have an emachines t2482 but i changed the video card,memory,CPU,and motherboard

same 11 inch case though

Yes an 11 inch tall case may be called a "mini tower" but dont confuse it with "form factor" they are the cube-like cases.