Initializing variables

[asha g]

I have a program with several variables - all are vectors or matrices.

Now do I initialize the ones I need to save and plot at the beginning of the 
program  or all the variables that are used in the program?
eg - if I need vvvv and abhh do I just write:

vvvv= zeros(niter, N); abhh = zeros(niter,N);

before the start of the script file or shd I do that to each one of them. When 
niter is large and N is large, this takes up lots of space and slows down 
operations. However, when I don't do it to all the variables I am not sure what 
is going on especially as there are many loops inside.

I hope this is clear.
Thanks
Asha