I am making a PCB layout in Altium Designer. I have selected "Force complete tenting on top and bottom" for vias. Does it have a disadvantage or more expensive? Tenting seems better but i wanted to be sure. Thank you so much.
Full tenting is normal. You will be fine if you fully tent all vias. It will not have any impact on cost.
Why would anyone ever leave vias untented?
Well, sometimes people leave some vias untented on prototype boards to facilitate probing signals. But vias near pins or pads should always be tented. Vias which are underneath components cannot be probed or visually inspected, and so they should always be tented, too. If you want to leave other vias untented, that is up to you.
Ask yourself if you need to access that via for reading a signal. If no, tent it. If yes, un-tent it.
About the cost, it can be simply understood as allowing solder mask over the via (tenting), or not allowing it (untenting). The electrical connection on the PCB will be unaffected but your access will be affected depending what you choose to do with the via. Also, this would do nothing to change the cost involved.
PS: I've seen some cases where you can use untenting to provide heatsinking.