I order from NE just like I order from everyone else... based on total price, including shipping or sales tax. If it's a big ticket item (to me that's pretty much anything over about $150) and it's an item that may be subject to a warranty return, etc, that is factored in as well. If it's just parts or something, NE looks pretty good... no sales tax, usually free shipping. I can't justify driving down to Dallas just to buy a case fan from MC... I would spend $20 in gas to do so.
It's also a trade off... I'll be getting a new case next month, it's $99 at NE and MC... MC I pay sales tax (8.25%) plus gas, NE I pay $12.79 shipping and it's setting at my door in 4 or 5 days. MC would allow me to pick out the box, NE box may (I figure 50/50 chance) be damaged in transit... and no etailer is immune to this. Amazon is on my poop list because they caved and universally charge sales tax, although I ordered something from them last night... as opposed to ordering it from my distributor for less money + shipping, but no sales tax.
It's all about what it's worth to you.