Should i change my front tyre size?

[hazza_ek]

i would like to change my front tyre size due to needing new ones anyway.
 
 
at the rear im running 255/40/17 9j
at the front its 235/45/17 8 j
 
i would like a bit more stretch and lower profile so wat would you lot suggest?
 
i know the way the tyre sizes are now is best but surely if i put a 215/40/17 or 225/40/17 on it its not gonna make much differance to the rolling radius that every1 keeps harping on about?
 
would it be safe to use either of the 2 sizes above or am i missing something with tyre sizes? lol
 
thanks
 
harry