Is corruption/crimson spreading something I should worry about? [duplicate]

Solution 1:

If you haven't yet progressed to hard mode through defeating a certain boss (which you won't meet unless you dispose of a certain item using a specific method), you have nothing to worry about.

Before that, corruption/crimson spreads very slowly, only on the surface and is blocked by sunflowers in its path. So you don't really need to worry about the spread in this phase of the game, because sooner or later it will get blocked anyway by natural obstacles.

However, after you defeated that boss, the spread of these biomes becomes a lot more aggressive. It spreads faster and through a lot more blocks, including under ground. And even worse, a block can infect any block in a range of 3 blocks instead of just adjacent blocks. That makes it a lot harder to contain the growing biomes in this phase of the game.

More information can be found on the Terraria Wiki article on biome spread and counter-measures are described in the guide on maintaining world purity.