When, if ever, is it a good idea to buy 1-Rune cards?
Almost always. If Chapel in Dominion has taught you anything, it is that trashing the chaff from your deck makes you much more likely to draw the good cards in your deck. For this reason, you should almost always buy Void Initiate. This is especially true early in the game, when you have a high density of Apprentice & Militia cards that are the polluting your deck (which are also worth 0 Honor).
For similar reasons, you should almost always buy Arha Initiate. It gains you 1 Honor (half the cost of a Heavy Infantry, and a third the cost of a Mystic and replaces itself when drawn.
Lifebound Initiate isn't a bad card, but buying a copy isn't a no brainer like Void and Arha Initiate cards are. This card becomes much better with other Lifebound Cards in your deck, and other Void/(Enlightened) cards that strip your deck of the chaff/(draw more) and make you draw into your Lifebound cards more often
Mechana Initiate is the worst of the bunch. I would almost never buy this card. The flexibility of adding Runes or Power doesn't seem that great. Perhaps if enough Burrower Mark II (Draw 1 first Mechana) or Rocket Courier X-99 (When acquire Mechana put into play) and other Mechana cards are available in the center row, then I might consider it, but usually I would pass.
Regarding holding off on buying a Heavy Infantry instead of an Initiate to prevent an opponent gaining a better card
The only real way to calculate this would be to figure out how likely a good card would come up, when your opponent wouldn't have had access to the card if they had to buy the Initiate themselves. For example, imagine if our opponent has 5 Runes worth of cards in their hand and 2 Power. There are 48 cards left in the center deck, (one of each different name). Lets assume for the moment that all the cards showing in the center row have 3 Power or more, and are 6 Runes or more. By not buying the Initiate, you have the possibility of the new center card being a 5 Rune cost card, that your opponent wouldn't have been able to purchase if you hadn't removed the initiate from the center row for them. Since their are only 5 cards that cost 5 Runes ( Ascetic of the Lidless Eye, Flytrap Witch, Snapdragon, Watchmaker's Altar, Voidthirster) out of 48 5/48 = 10% of the time you help your opponent.
Technically speaking, 3 of those 5 cards only gain you 2 Honor anyway. If the game was close to ending, and no one would be able to shuffle anyway, those 3 wouldn't matter (they could have just bought a Mystic and Heavy Infantry instead). If you had exactly 2 Runes though, it would be more likely in this scenario that you would draw one of the 4x Initiates to replace the Initiate in the center row. So you should probably buy the Initiate anyway.