If your values to be used as indices into an array are the result of some calculation, especially one which uses division, multiplication and really anything more complex than addition and subtraction you should use:
idx = round(idx);
for an index value, idx, or just put round(idx) straight into the indexing line if you prefer.
If you are confident you should never get floating point-based index inaccuracies then you can, of course, leave that off, but if you cannot be sure of every conceivable output of your function that creates idx then this is best to ensure a valid index value.
