It all depends on your complete setup, how you plan to use the engine and what RPM range you want to run at.
Typically, relieving a block will not help lower-end torque or horsepower, will probably hurt it as you lower compression a bit. If you desire peak horsepower and plan to regularly ran it over 4,000 RPM, then it can help flow. Also, relieving by itself will probably not help a bit - so unless you do a full competition style port job and install bigger valves, probably isn't worth it.
What combination of parts in the engine (cam, intake, carbs, ignition, heads, compression ratio), what trans, what gear ratio and what are you looking at as far as usage and engine behavior.
Also, you might consider more of a 'step relief' or semi-relief, where you blend/smooth the valve pocket cuts into the transfer area to help the flow, but not hurt compression too much.
Best of luck - if you need more info, PM me.
Dale
Attached are some pictures of various reliefs I've done (full, stepped, semi) - mostly Ford, but also Flathead Cadillac:
Relief1 copy.jpg
DeckAtAngle.jpg
59L-Porting-Guides-Relief-80.jpg
ReliefWithRadiusBoreEdge copy.jpg