In 3.4 I was able to see border-less grid lines of tables in writer. Now in 3.5 they are invisible by default (Mint 12 using testing PPA for 3.5)
Since this is a StackExchange-style Q&A site where questions are placed in order to have a future reference I’ve decided to share where this option is, since I was lost in frustration of trying to find solution in the help, forum, here or googling it.
So, in order to make table gridlines (for borderless cells) visible/invisible you need to go to:
Tools - Options - LibreOffice - Appearance
And in the Custom colours list enable the checkbox next to Table boundaries and optionally select a colour for them.
There you go.