300 mcm T90Nylon is rated for 295amps. Using the 5% rule, one could use parallel 300mcm runs for a 600 amp service?

If your calculated demand is 620 amps, you can put in a 590 amp service, which would be fused at 600 amps according to 14-104, if a 590 amp fuse is not available. That is, you can install a 590 amp service and fuse it at 600 amps.

Generally if you are considering applying the 5% rule I would first run it by your local inspector. Yes, it is a rule but doing so makes for a better relationship, especially if you don't know him/her.