In 2012 Raghavan, Samuel, and Subrahmanyam showed that the Kazhdan--Lusztig basis for the Iwahori--Hecke algebra in type A provides a ``canonical'' basis for the centraliser algebra of the Schur algebra acting on tensor space. In 2022 the second author found a similar result for the centraliser of the partition algebra acting on the same tensor space. Each basis is indexed by permutations. We exploit these bases to show that the linear decomposition of an arbitrary invariant (in either centraliser algebra) depends integrally on its entries, and describe combinatorial rules that pick out minimal sets of such entries.