Why POD printer need 100% K black for text but our home printer don’t need to worry this?

Had to check out your Lato font before I could respond.

The good news is that it's a sans-serif font, so you don't have to worry about those little serifed registers breaking up or getting plugged up when you ptint with it. The not so good news is that yes, the strokes are thin for the relative weights of the font. And that causes challenges ...

To share a story with you, I used to work with a design and training company that used lots of a similar font, Avenir. Like your Lato, it was extremely thin for its relative weights. And this company expected us to use it everywhere — all our signage, our marketing collaterals and every piece of training documentation we ever produced. Heck, I think the owners of the company sprinkled it on their morning cereal. So I got some experience working with fonts like this ...

Like Avenir, it seems like your Lato Black seems about the equivalent of Helvetica Semibold. Smaller type sizes demand heavier weights; the bigger the type size, the lighter weight you can get away with reversed against photographic or color build backgrounds. If you need variations in your reversed type, vary with italics of the same heavier weights rather than varying between weights of the font.

Don't worry about overprinting. If you're using black type, it generally goes down last — CMYK doesn't only describe the colors used in four-color process, it also reflects the normal order of laying those inks down on press. Reversed to white, as you've described it, is simply "printing" with the absence of any of those inks. And finally, if you're determined to use rich black you're going to need tight registration — over full-color images your rich black builds will float with your image separations on the color plates,  and will show color highlighting evidence of the crime in colored edges against your black gradient. In short, you're going to win ... or you're going to lose.

Hope this helps,

Randy

You do need to worry about this, yes. CMYK printing to a press gives full control of ink levels, you get what you ask for. Home printing is RGB based with the driver converting and correcting to meet average needs - unacceptable behaviour for professionals who don’t want average. 

Because Microsoft Word isn't set up for process color unless you use a different color than "Black." By default your "Black" in MSWord is 100%K.

Using rich black — Key color blacK with smaller percentates of Cyan, Magenta and Yellow — requires close registration laying down four color builds on press. Even the slightest misalignment will result in poor edge definition (fuzzy type) using rich black for small type and thin lines printing rich black. At worst, you're looking at pseudo-stereographic effects when those plates misalign and CMY color builds float away from your black print run on press. Take a close look at all the color images in your Sunday paper and you'll undoubtedly see examples of the negative effects of press misalignment, somewhere, in the press run.

As for your printing small white text (actually, knocking out, as in the absence of printing those letters if you're not using white ink) I don't think you're going to have to worry about type "ghosting" so much as type breaking up and disappearing into your black gradient. Depending on your POD process (ink/inkjet/toner), you're probably going to need to use fairly large type sizes — and sans-serif type because those little serifed ends will be the first thing to break up around the edges of your characters, especially in small sizes — to get good results with printing reverses. Especially against gradients.

So if you're asking me, yes, you definitely need to worry about this. This isn't extra work, this is necessary work. Because with Print On Demand, your first proof of the job will be your print run. And if you don't meet your printer's stated specifications, it's all going to be on you and you're going to eat that print job.

Please take this to heart, for your own sake.

Randy