In answer to your question's yes, no, no. Now the details. It is your work you can do as you like. Generally as lines converge in shading this is what gives the black tone. Generally there are enough lines to accomplish this. It would probably be best to use lines to accomplish this. Often I use cross hatching if I have improperly converged the lines by falling into a rut in my shading lines or there are to few lines.
One of the main problems is the scope. Check with the naked eye and you may very well find you are black enough. Remember how the engraving will be viewed. Hope that helps Fred
The good news is that you can have more control with shaded tones with dots than you do with lines.
The bad news is 1.) dot shading can take a very long time compared to lines, and 2.) is extremely delicate and easily damaged. Also, 3.) dot shading looks fabulous in photos when lit correctly, but can be extremely difficult to see if not lit correctly. As SamW said, a combination of lines and dots can work really well.
The engraving below is by Simone Dianelli of Florence, Italy. It's 100% line engraving and stands as proof that even the most subtle tones can be achieved with lines if you know what you're doing. I would have bet a paycheck that this was dot engraving, and was astounded when I examined it with a loupe and discovered it was all lines.
Well Sam, it's been awhile since I saw this, but as I recall it was extensively crosshatched and not dot shaded as I would have expected. I wish I had a really high resolution image. I sent an email to Simone so maybe he has one. Martin Strolz might also know because he introduced me to Simone and it's possible Simone engraved this when he was studying with Martin. Not sure about that though.
I am a newbie engraver too, but....
since I used a combination of lines, crosshatching, and dots in scrimshaw for 20 odd years, I naturally tried it on metal. I am about half way done with a pistol in that technique which I'll post pics of when I'm done. Personally I kinda like it but I'm not a pro.
Dang, I thought I had an original idea there for a minute.
For all-dot bulino engraving a square graver with 45° face and no heel has worked very well for me. I don't think a dotting graver's geometry is very critical as long as it's a reasonably narrow V (I wouldn't use a 120° because it's too wide).
For bulino, a square graver appears to be what Phil Coggan uses; Ray Cover uses 80 degree. I find that 105, 70 and 50 also work. Lee Griffith did some amazing bulino with a 120 a few years back. It appears that talent, practice and concentration is more important than geometry.