I suppose it depends upon how ultra-cautious (or 'risk-averse') you are. In a domestic environment, if the MCB is locked off and proper 'testing for dead' has confirmed that the circuit is 'dead', then any residual risk is incredibly small - many/most would probably say 'negligible'.
Any theoretical risks necessarily relate to cross-connections of one sort of another between different circuits and, not only that but, if the circuit initially satisfactorily 'tests dead' then the effects of such 'cross-connections' would have to be 'intermittent' (or, as below, only present if something was cut/disconnected) if they were ever going to present a risk.
Probably the greatest potential risk is that of a neutral being shared between circuits (a 'borrowed neutral') - which, if cut/disconnected (on a 'dead' circuit) can create a risk under some circumstances. Since that is something which, if it exists, is almost always (in domestic installations) between two lighting circuits, then one might regard it as wise to lock off the MCBs of all lighting circuits when working on them. However, if you have two lighting circuits protected by different RCDs (or RCBOs), then a 'borrowed neutral' cannot exist without one or both of the devices tripping - hence giving reassurance.
I would imagine that very few are as ultra-cautious as SUNRAY suggested.
Kind Regards, John