If it's FMS speed reduction from M.079 to M.076 it's unlikely that it'll happen all at once, so no need to say anything...
Except in 'specific environments' such as oceanic, filed mach numbers/speeds will be not even noticed by the ATC... Most plans have a TAS speed anyway and very few ATCs will be accurately predict what that means relating to Mach.
However, most ATCs are very good at 'predicting' what speed a particular type will be flying... So unless you want to fly outside a 'normal' profile such as slowing up for a slot time/curfew time etc then we probably won't notice/care. Where we need a specific speed we'll let you know... As a rough guide if you are changing M.03 (in one go) or more let us know ASAP...