New shower thermostat mixer bar

No you shouldn't need to, the hot and cold feed to your shower should have individual isolation valves on there supplies, they are usually ball a fix valves and can be isolated by turning the screw half a turn. Alternatively you could isolate turn off the gate valves, the ones with the red wheel handles supplying the hot and tank fed cold supplies, you must make sure the pipes are flushed out before fixing the new valve in position. Why not get some no obligation quotes from the my builder plumbers, good luck