The bread will block the flow long enough for you to solder the fitting, and it will disintegrate when you turn the water back on.

It's compounding effect of multiple strategies, multiple actions, repeated with regularity and persistence.It is about building a pipeline BUT every foot of pipe laid requires regular maintenance or else it disintegrates into nothing and is gone.

@Andrew Coulton You forgot to include lead paint, asbestos popcorn ceilings/walls and disintegrating iron cast sewer pipes, among other pleasures that come with structures built prior to 1980.​As for foundation, there is more to foundation repairs than just the foundation.

Then listen to what Bill says above...As long as everything is functional and not putting the property or tenants at risk (nothing is about to leak, explode or disintegrate), hold off on the capital repairs until they need to be done.The one exception is stuff that would be a PITA to repair while a tenant is in the property.

Old houses have cast iron pipers that will disintegrate (because of age and/or foundation shift).

The particle board disintegrated and is falling apart.Best solution is to remove the vanity, pull out the bottom, and replace it with a new, solid board.An alternative is to just replace the vanity.

The filter was literally disintegrating in my hands because it got so wet.

Old houses have cast iron pipers that will disintegrate (because of age and/or foundation shift).

Regional banks would disintegrate, simply put. 

We don't need this thread to disintegrate; it's a good discussion thus far.Back to the original question: IMO there's no right or wrong answer.