I'm a tinkerer, by nature, especially when it comes to building my own constructed lists or refining decks I dredge up from the corners of the internet. In general, many competitive Magic players are constantly adding and removing cards from builds they find through coverage of professional tournaments.
The thing is, though, this is rarely the best course of action. I never fully appreciated how well-tuned most winning decklists are until I finally gave up messing with what was working.
The truth is that a deck that takes down a Pro Tour is a honed machine, comprised of dozens of parts, assembled in such a way that it can repeat the task it was designed to do as many times as necessary. If you imagine a deck like an engine, you'll understand that unscrewing one nut could lead to one hell of a Michael Bay style explosion.
When you are tempted to add just one Grave Titan because it should theoretically be good in the metagame you are expecting, you need to remember that you are taking something else out that had a purpose in the first place.
It can be even more problematic if you were not part of the team that originally built the list, and you swap in Rune Snags for Mana Leaks because of "the late game." Sure, you might get away with it, and sometimes, your change may even end up being for the better, but more often than not, you just messed up the deck's basic plan.
The tip of the day is to be careful when you are re-wiring the security system of Bill Gates' house, you don't know which wires control the pirate-ninja robot butlers.