tension doesnt effect good players.
keeping peices theory

It's not good advice to arbitrarily keep pieces on the board. You should exchange if it does something for you, and avoid making the exchange otherwise. Technically won endgames can appear with many different types of diminished material and is the reason why endgames are important.
One endgame tip I read (... somewhere) was from Korchnoi, saying as a rule, you should leave pawn tension in the endgame unless exchanging to relieve it definitely does something good for you, or definitely prevents something bad from happening to you. He was criticizing one of his own moves (made in his youth, as a young master) where he arbitrarily resolved the tension which he said is almost never a good idea.
As for the ability to keep tension on the board and play a good move, that's probably good advice for anyone under master level, not just beginners.

When I was just starting out I discovered ideas that let me beat other weak beginners, but apart from that those ideas were useless. That's to say they were in fact useless.
Better to study sound fundamentals that already have the masters' stamp of approval. This way you won't have to unlearn faulty concepts that are only going to trip you up later.

One condition in which it is advantageous to simplify, is when you are up material. The idea is, that you keep trading off until your opponent has no pieces left and you do.

If maintaining tension and keeping pieces on the board is favorable to one side, resolving tension and exchanging down must be favorable to the other side.
The important thing is to figure out which you are!
lol, well you just completely busted his idea nice :p

bah! keep as many peices as possible... in the end, you dont want to be the one with just pawns and a king.

Oh hell, better for some to not even start a game since they think their beginner ideas are better than any distillation of more than 100 years of master praxis.

its nice to be part of a forum where people try to be helpful.
but honestly, zingrat, the only thing useful about your advice would be if we could convince our oppenants of it.
YOU WILL BE A BETTER CHESS PLAYER IF YOU LEARN TO ABANDON SILLY RULES OF THUMB SUCH AS THIS ONE.

Perhaps zingrat means you should not blunder away your pieces for no compensation. That's probably pretty good advice.

for what it's worth, Mikhail Tal said that when he considered any position he first considered what sacrifices may be available to him - the way I understand it he was a pretty good player

One can not have overall rules like "exchanging pieces is bad". Every exchange must be evaluated on its own merit. There are many times when an exchange, even if the material is even, is useful. Some examples-
Exchanging minor pieces when the position makes the opponents piece stronger. Chopping an outposted knight with your bad, inactive bishop is a good idea.
If your opponent has a strong attack, sometimes exchanges can deflate his initiative.
If your opponent has a space advantage, sometimes exchanges can give you more room to maneuver.
Exchanges tend to favor the side with more material, as you are threatening to reduce down into superior endgame.
Exchanging off an opponent's defender can give you control of a key file, diagonal, or square, even if you don't gain material out of it.
Like everything else in chess, you have to check the tactics, and then picture the resulting position, and try to determine if it is a better or worse position that you have now.
try to keep as many peices as you can throught the middlegame. you will need them in the endgame!!! dont have a big, bloody war at the beggining of the game, try to avoid fights, and take advantage when your opponent makes blunders. in the end, you will have a mass advantage.