problems only arise through inexperience. 99% of the time a plumber wont end up with leaky rads after the event because he knows what he's doing... also 90% of the trv's I encounter wont be completely shut off anyway i always carry some sort of blanking device with either a 15mm nut and olive or a 1/2" tap connector with a rubber washer sort of thing..
leaky trv's arent really the problem, its puttin em back on when the real trouble starts..
the easy answer is to wrap plenty ptfe round the olives for the 15mm tails and use hawk white or boss green on the flages of the old 3/4 valve tails.. dont over nuke the nuts job should be fine in 90% of cases, slight leak give it a further nip.
they can leak from the pipe end too though (pays to be gentle when taking em off) and you can take your pick from pipe freezing, quick wrap some ptfe round the olive while you get a little bit damp (takes 1 to hold the valve and 1 to wrap the tape so its 2 of you getting wet really, and relieving the system pressure helps a lot here :
) or drain down the system...
or if its an old gravity give yourself at least half a day to bleed the system back up
risk and reward innit... if you know what youre doing the risk is minimal and it could easily be the difference between getting and losing a job?
if you dont know what youre doing then the risk is likely to be extremely high and the possible consequences dont bear thinking about... ive had the 'trv open at 5am' trick happen to me.... on a gravity fed system... luckily it werent my name on the invoice
another little tip for 'rounded off bleed valves' on real old rads - a mini cold chisel or even an old 1/2 wood chisel can be used to carve a little slot in the end and you can get a screwdriver in.. the combination of shocking it with the chisel and a screwdriver usually works.. careful though...