The reasonableness of this assumption probably depends on the environment around the sample.
A sample that is sealed from the surrounding environment is more likely to avoid contamination or loss than one in an open environment where materials may be carried into or out of the sample by water or simple diffusion.
Carbon-14 dating depends on the amount of atmospheric carbon-14 relative to carbon-12.
The geophysical reservoirs include the atmosphere, the oceans, the biosphere, and the sediments.
Violations of this assumption occur in the ocean reservoir due to the time required for mixing of surface waters with the deep layers.
While the small variations in isotope decay that have been reported may not invalidate all isotopic dating, they raise questions about the assumption of completely uniform decay rates.
A second assumption is that the sample being dated has not experienced any loss or contamination of C over its history.