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Whether you need to shape depends much on what your provider does when your traffic is over contract. If they drop it, then it often is a really good idea to shape it to the contracted value.
Assuming the provider enforces CIR for all traffic, then you want to shape for all your traffic before you prioritize it. This can be accomplished using a hierarchal policy.
e.g.:
policy-map shape_4_CIR
class-default
shape average 50000000
service-policy outbound
policy-map outbound
class high
priority 6000
class medium
bandwidth percent 40
class low
bandwidth percent 30
class class-default
bandwidth percent 20
When shaping, often the shaper only accounts for L3 but the provider accounts for L2, i.e. you need to shape for the L2 bandwidth. Since the L2 overhead varies per L3 packet size, you can either shape for worst case or your expected average. Generally shaping 10 to 15% less than nominal bandwidth works well. So, for example, in your case of 50 Mbps CIR, you might want to shape at 45 Mbps.
PS:
If your priority class is really sensitive to timing, since the shaper imposes some overhead and bursts (the latter can be minimized by reducing Tc), you can move it to the parent policy and shape such that there's a "reserved" block of bandwidth. (Also, you many need to decrease Tx ring limit.)
e.g.:
policy-map shape_4_CIR_less_LLQ
class high
priority 6000
police 6000000 #NB: LLQ implicit policer won't see congestion
class-default
shape average 44000000 #NB: didn't allow for L2 in this example - but even more important when timing critical
service-policy outbound
policy-map outbound
class medium
bandwidth percent 40
class low
bandwidth percent 30
class class-default
bandwidth percent 20
