What happens when you paint underside of deck? I have been advised not to do that. Can anyone give a good answer?
In my opinion you should never paint a deck it will only seal in moisture and prematurely rote the wood, painting the bottom even seems worse I don't think it will dry out, only use a stain/preserver
Things don't dry out well in the shade. The prolonged moisture tends to cause peeling, blistering, etc. Use a penetrating stain instead unless you're in a very arid climate.
Does it help?
Yes, if your deck is more than a few feet off ground for sure.
Is it worth the time?
People complain all of the time about painting decks. If you use the right products and maintain the deck even cheap lumber can last a good 20-30 years.
I build decks all the time and paint my own. I spray on a oil based primer in a very light coat. If I am building the deck, we spray the decking before it is screwed in so we can flip the boards (spray, flip 5 hours later, spray). But you can certainly spray deck with a light coat - top, cracks, and bottom - when it is warm outside. You are only limited by the amount you can apply at once.
After that we hit it with an oil based paint. *I have tried oil plus epoxy and it helps only minimally. The underside is important because the water on your deck will dry on the top and run and hang onto the bottom. Expect to have to repaint the bottom faster than the top. But again with maintenance this is an easy job. It becomes harder when you let the paint get to the point where it is cracking everywhere (oil outlasts latex by far).
It is more about your ability and expectations. If you hit the problem areas every 3-4 years and expect a new coat every 6-7 your deck will last a long long time. If you aren't going to do that then just don't paint it to begin with, throw down some sealer and expect to replace your deck much sooner.