Wikipedia has a page on tipping and gratuities, and claims:
Service charges are included with the bill, and tipping is uncommon
However, their link is broken and I suspect it might be wrong. All other resources I can find online indicate that you should, and how much.
10% in restaurants and taxis. $.50 - .75 per bag to porters
In Paraguay, restaurants that do not automatically add gratuity (it
will say if they do at the bottom of your menu) then a tip of 10
percent is enough. I found that my servers delighted in the other-
than- monetary- tips I gave them. Paperback books, picture books
(postcards) and compliments to their managers were high on their list.
In taxis no tips are necessary. The standard of $.50-$.75 per bag to
handlers was the norm.
10% should be left for the waiters. Small change should be given to
porters and taxi drivers.
So perhaps the culture has changed, but all signs are indicating that yes, you should tip, and especially if the restaurant indicates as such.