As mentioned, it depends on the combination.
When I first start tuning a new engine at the track, I use high octane fuel and slowly increase the total timing till the performance levels off, then usually go back to the previous setting.
If it is a pump gas engine, after setting the timing as above, I will slowly reducing the octane rating, usually by mixing pump and race gas in various ratios while putting miles on the engine. Normally a performance timing curve works fine, but sometimes, with a truck or car with low stall (operates alot below 3,000 RPM), I will have to slow down the advance curve so total timing comes all in at a higher RPM.