That's a tough call. If you are putting stiffer springs in you may not have enough rebound damping unless you modify the shim stack on the stock rebound valve or replace it with a better valve.
I know the 04 750 rebound valve was ok but I'm not sure about the 03 stuff.
I'm not exactly sure what the stock oil weight is to tell you the truth. It seams like most suspension guys are using 5w oil. That's what I'm using in both my 01 750 race bike and 04 750 street bike. Both have had the internals replaced along with stiffer springs.
Please provide a bit more information on what you have in mind to do. Race/street/trackday/dual purpose? Rider weight?
Also try this before you change anything. Turn the rebound adjusters all the way in (top adjuster) and, while holding the front brake, push down on the bike hard and let it come back up on it's own. If it's really slow then you may be ok with the stock valving. If it comes up fast and bounces or pogo's then internals would be a good idea. Heavier oil is a band-aid fix and only masks the problem.
Hopefully Leanangle (Jody) will get in on this for some input.