Another Z Axis question

In researching, I've notice that a lot of people have mounted their motors to the outside of
the mill, where the handle used to be for the Z axis drive. In this configuration, you're
dependent on the gears that drive the screw. In my case, these are bevel gears, and would
provide an unacceptable amount of backlash (so I'm led to believe) - In the applications
where this has been done are helical gears used? Do helical gears provide distict
advantages, as far as backlash, over bevel gears? If so, would it be prudent to attempt to
retrofit the existing bevel gears, with helical replacements?

Thanks for the info,

Christopher