doughayman,
Welcome to the forums, by the way.
A couple of points that might be worth noting.
Firstly, always put your 3:4 (600x800) cover in the *.htm file of your epub. Try to avoid adding a cover via calibre for the reasons noted below. TIP: If it is a jpeg, don't keep resaving it while you tweak it, there will be a small, but cumlative, loss of quality.
Secondly, ensure, especially if you have changed or updated your cover in the source epub, that the correct file association is in the content.opf file. ("Tweak" option, if you right-click on the book in calibre, will let you examine these files, check the name is still exactly the same, and watch case sensitivity.) Calibre will still draw on the cover image file, but it may occassionally show incorrectly on the bookshelf.
Example of all you really need.
Your *.htm file (Cover image page)
<?xml version="1.0" encoding="utf-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta content="your own info here" name="Your book file name here" />
<link href="../Styles/your own.css" rel="stylesheet" type="text/css" />
<title>Your title here</title>
</head>
<body xml:lang="EN-GB" xmlns:xml="http://www.w3.org/XML/1998/namespace">
<div style="text-align: center;"><img alt="" src="../Images/Your cover file name here.jpg" /></div>
</body>
</html>
In the content.opf file look for the cover association entry. It's the last line in the metadata section at the start of the file.
<meta name="cover" content="EXACTLY AS NOTED IN YOUR HTM FILE.jpg" />
Now, this is where most folk seem to have a prob. If you import or adjust your cover in calibre library, that image seems to be the one used during conversion...
gremlins rub there hands together and get to work.
If your opf association and cover image are all good, the cover should appear in the calibre library, on import, exactly as it is ratio'd (I know, not a real word) in the epub file. During conversion this file is used, unaltered.
Thirdly, each device will resize your internal cover to suit it's screen, so ratio is the critical part if you want to avoid borders. Default for Kindle e-ink, enlarged or centered for fire and apps. Note, previewer is a simple emulator, and while it functions just fine, mostly, it is no substitute for a physical device.
And here's the bit I think wil put your mind at ease. When you upload to Amazon via KDP your file is run through a conversion process, even if it is in mobi/prc format. Uploading a mobi/prc is probably the preferred way to go nowadays, as it suffers almost no alteration from your source file, but it is still "converted". This conversion process will almost always overcome any "glitches" you see in previewer. (A common one is right and bottom edges of the cover cut off slightly.)
In conclusion, if your image is the correct size/ratio and is associated correctly in the opf file, it will show and convert fine in calibre. You can upload your mobi to KDF, convert, and download a preview copy without going live (i.e. stay "Draft") So no need to worry about "half-assed" publication. You can upload as many times as you want before you go live, and even after you go live, once you spot that blatently obvious little error you've missed twenty times during proofing...
Essentially, if your file is good, don't sweat it. Looking for bugs that don't actually exist is a really crap hobby...
Good luck, and always tweak/test with a copy file, just in case.