The answer to your specific question is that you should always hit a soft 18 vs. an ace when playing a shoe game (which assumes that more than 2 decks are in play). In fact, it’s okay to play by the strategy of always hitting soft 18 against 9, T, and A, regardless of the number of decks (even a single deck), although there are detailed exceptions if you want to play perfectly.
In The Theory of Blackjack, Peter Griffin says to always hit soft 18 vs. A, except stand in 1 or 2 decks if the dealer stands on soft 17, with the exception of hitting specifically A7 in 2 decks. So breaking that down, you stand with A7 vs. A only when playing a single deck where the dealer stands on soft 17. In all other instances, you hit A7 vs. A (though you also stand on combinations of three or more cards that make soft 18 in double deck).
For those of you who, after reading the preceding paragraph, just swore off learning blackjack basic strategy, don’t. Playing by the basic strategy is a must for anyone who wants to play blackjack at any monetary level. Doing so won’t (except in rare situations) give you an advantage over the house, but it will get you closer to breakeven than you can get, with the same amount of effort, in almost any other gambling game.
Understand that the soft 18 vs. ace example is one of the more complicated rules in what’s known as the full "composition-dependent" basic strategy, which considers the specific cards that make up your hand. All you really need to know is the regular "total-dependent" basic strategy, which doesn’t consider specifics and is much easier to learn.
For purists who want it all, Griffin’s The Theory of Blackjack provides the most detailed basic strategy delineations available anywhere (we used the book’s tables to formulate this answer). For the other 99% of you, the book Knock-Out Blackjack provides the easy-to-handle approach of using a "generic" total-dependent basic strategy.
K-O authors Olaf Vancura and Ken Fuchs write, "While most basic strategies are created to apply specifically to either single or multiple decks, in practice, we have found it best to memorize one generic basic strategy table. This is because the gain in memorizing additional variations is very minimal, amounting to a total additional gain of roughly 0.03% overall." Memorizing the easy version takes about four to eight hours of study. In return, your blackjack skill level will rate above all but the proficient card counters.
