Certainly it's true that many wineries can increase the number of emails they send to people who have asked them to do just that (by signing up for their newsletter). 
 
If they can come up with compelling offers it won't offend people, but they can also add some interesting content to each email so people may even look forward to receiving them. 
 
One thing that wineries can do is during growing season send a picture with each email showing how the vines are progressing (and perhaps include a link to all the previous photos of the vine on the website). 
 
During dormant season photos of the crowds in the tasting room, the bottling line, racking the barrels, etc. can be used. 
 
It does make the email more interesting and keeps people engaged with your winery.  
 
There's also nothing wrong with putting these photos up on Facebook or Twitter.  
 
If your offers are good (new wines, special pricing), people will look forward to them.  
 
The frequency can be expanded slowly until you find the right level for your customers.

Thanks Larry for the comment. I find most wineries a bit shy to send emails frequently enough.