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Hey there. I'm watching someone rebuild a GSX-R750 engine and he mentioned that, in the service manual, it instructs not to lap valves but instead cut them to a "velvety but not highly polished finish" which will then provide a surface for the valves to "seat" into. I believe he also mentioned that the bike has titanium valves; I'm wondering if, when I can and do rebuild my 750's engine (which is vastly different from this GSX-R engine) this is something that applies (as in, it's a development that would be good to retroactively apply to my engine) or if I'll be fine lapping with grinding compound.

I think the first questions you may  need to answer are after you tear down your motor and can inspect its condition .
Depending on what you find , then you can  make a plan . Obvious , YES.  
 But I say this  for the longevity  of the motor  into the future .
   Point ,  Why cut into the seats if not needed . Sinking the valves  which in turn causes more work
I agree to do all work needed  , but  why cut life out of the head.
I was able to just lap my valves on the last rebuild saving time , money and  not reducing the life of the parts

    Dave

When restoring my 750, which only had 10.8k miles on it, the valve to seat contact pattern was pretty crappy.  The contact margin was okay on one side but really wide 180 degrees away.  I used Neway cutters to dress the seats, removing the minimal material necessary to clean them up and create a proper contact pattern with the valve.  The valves themselves needed a fair bit of clean up too.  I took them to a machine shop that has a really nice centerless grinder for valves and the machinist guy had to take off more material than expected to get them all squared up.  He tipped the valves at the same time to compensate for the expected sinking of the valves.  Reason for mentioning this is because a simple lapping job wouldn't have renewed the head properly.

Hopefuly I’ll be moving onto the head while the crank is out. I doubt I’ll take on valve work myself but I have a question. Is the trueness of the valve seat cutter dependent on the condition of the valve guide?

wrote:

Hopefuly I’ll be moving onto the head while the crank is out. I doubt I’ll take on valve work myself but I have a question. Is the trueness of the valve seat cutter dependent on the condition of the valve guide?

There is a tapered arbor that wedges into the guide so even if there is some wear the arbor will center itself.  That said, if the guides are sloppy it's best to replace them before moving to the seats. If you have a dial indicator it's pretty simple to perform the "wobble test" to judge guide condition.

I had Larry replace my valves and springs 50,000 miles ago and last winter I decided to install new valve seals and clean and lap the valves.
The seals were hard and the valves were dirty but the job went well and I'm glad that is all I did to the head, working fine.