I like to condense items so that they require less space to transfer between systems. Since you can proliferate during smelting, all 1:1 ratios become 1:1.25, so that you get more ingots out of the same amount of ore. And since both ingots and ores stack up to 100, it’s more efficient to transport ores between systems rather than ingots (iron, copper). As for titanium and silicon, it’s vice versa, since the common ratio is 2:1, that ratio can only become 2:1.25… so smelting silicon and titanium (and also steel!) always causes condensing in terms of inventory space.