At handwriting, do all natives really write letters with their accent marks on them? And top down question marks and exclamation marks at the beginning of sentences? Or do they simply omit them ? Like á, é, í, ó, ú, ü, ñ, ¿, ¡
Just as some people write "because" as "cause" or "cuz", it's impossible to answer this with a Yes or No. Some people do, some people don't.
Also, as digital has become the most popular medium for communication, people need to actually handwrite less and less.
So, I can tell you just what I can see. Most people don't use accents, however it's still viewed as important in business, education, settings. You're viewed as more educated if you do.
You must know that accents aren't just extra symbols. A word with an accent will mean something different from one without it. Using them makes it easier to understand what you really mean.
At handwriting natives use to write letters correctly.
When in mobile, they hardly ever do. They almost always omit them.
It's considered 'slang'-ish.
I'd like to answer this question with more than anecdotal information, but for the most part that's all we have. In my experience, people nowadays write in longhand using the proper accent marks and punctuation, but tend to leave that out when typing, except when the occasion calls for it (e.g. when typing a CV or a formal letter). This is of course if they learned the proper orthography in the first place.
Leaving out the accent marks seldom produces confusion, but it may pose a problem in some cases. Sí ("yes") vs. si ("if") is one notorious case.
Spanish-language natives recognize ñ as an individual letter distinct from n; the tilde ~ over ñ is not counted among the accent marks, and is never left out.
The diaeresis over ü is rather rare and mostly not essential for understanding; I'd say (but I have no hard data, of course) many more people leave it out in handwriting than they do the accent marks.
Research has shown that handwriting helps children identify and produce letters; this is probably because of what is colloquially known as muscle memory. When we learn to write by hand (first single letters, then whole words), our brain keeps a memory of the movements and gestures, the starts and stops, etc., which is why in time each of us develops a distinctive handwriting. This is not the case with typing, since every letter is produced using more or less the same movement (i.e. depressing a key or hitting a small area on a tactile screen). Maybe this is why we keep using the proper marks we've learned as children when we write by hand.