Inquiring minds want to know...

No.
Yes.
Yup.
 
;-)

I do all of the above. I use them, I tweak them and I make my own. I rarely save one though. I look at it as one of a kind. It also depends greatly on what plugin I'm using. 

sometimes

My take is sometimes a preset gives you a good starting point without lots of brain pain.

However software synths are quite easy to tweak, well most of mine are, and they are just begging to be tweaked, which makes me think it would be insulting to the creators not to tweak.

Of course the Modular synths Softube Modular, Arturia's Modular, ARP 2600 etc are there to build your own.

Consequently my answer is.

Do You Use Presets As Is? Yes as a starting point.
Do You Tweak Them? Yes every time, even if it is just to change a filter or FX to help it sit in the mix.
Do You Create Your Own? Yes with some of my synths.

I do all of the above as well. But when using signal processing plugins, I pretty much never use (or save) presets because compression, eq etc. settings on one track in one project will never be exactly the same as what's needed on another track in another project. Instrument plugins and some signal processing plugins (chorus, flange, reverb etc) are a different story though. I often browse through presets, sometimes use them as is, often tweak them and start from INT and bake my own if I have time and am feeling creative.

I use presets as a starting point also.
I create my own when I'm working on a multi song project with similar instrumentation to keep consistency between the tracks.  

I'm not usually inclined to. Every sound source is different in my ears. I like to start from a clean, flat slate.

Same as Joe.
 
I also use a few plugins that have so few controls presets make almost no sense, like Waves RVox or Cakewalk CA2A.

For certain things I will just grab a preset as is - like a reverb.  I might tweak it if something is off with it.  For other things like my vocals, I will have a preset EQ setting to pull out that horrid nasal sound and a preset De-esser setting.  Sometimes these need minor tweaks.
 
For everything else, I will start with one and tweak it, or just start from scratch (EQ & Compression).