series motors starting torque is very high & hence it should
be started with load only.shunt motor starting torque is
marginal and hence it should be started with least load.

in series motor field winding is series to armature winging.
at starting condition we control starating current for 
prevent the winding from over current.so flux 
low.N=1/flux.so motor rut at dangerous speed.to avoid that 
start with load.
 in shunt motor field winding is parallel to armature 
winding.the field is current is mostly constant.so flux 
also constant.sppeed also constant

if series motor starts without any load,ie open circuited 
condition...a very less flux will be generated that is 
mainly due to residulal flux and hence speed being inversly 
propotional to flux,a speed (theoritiaclly infinite)will be 
there at starting and hence its not started at no load in 
the begning.
there is no such compulsion in dc shunt motor but running 
it on no load in starting ensures that the good amount of 
working flux will get generated before load is applied.

in series motor there will be no residual flux. so there it 
have high starting torque,which will lead to to dangerously 
high speed.but in shunt motor there is no such condtion