Here are some quick tables to help you understand when it's a good time to gamble on silver arrows being in Level 9.
To understand these tables, first you must remember that there are five items that can never be in Level 9 (unless the flag to allow important items into Level 9 is selected) - the Bow, the Ladder, the Recorder, the Power Bracelet and the Raft.
Taking this into account there are now 10 of the 15 items that can appear in Level 9 - Boomerang, Magical Boomerang, Book, Red Candle, Magical Key, Red Ring, Wand, Heart Container, White Sword, Silver Arrows (As of this time, Magical Sword is not able to be shuffled, so it doesn't count towards this number).
With either white sword or ladder heart are shuffled, only 9 of the 15 items can appear in Level 9, since either the white sword or ladder heart would be known to be in their normal locations and not shuffled into the mix.
With the white sword and ladder heart NOT shuffled, only 8 of the 15 items can appear in Level 9, since white sword and ladder heart would be known to be in their normal locations and not shuffled into the mix.
As you start to discover the non-important items (Boomerang, Magical Boomerang, Book, Red Candle, Magical Key, Red Ring, Wand, Heart Container, White Sword) in Levels 1-8, your odds of the Silver Arrows being in Level 9 increase. Finding one of the items that is required to be outside of Level 9 doesn't change these odds, as it has already been calculated in from the beginning.
The other unknown benefit to exploring Level 9 for the Silver Arrows is that you map out the level and also have a chance of running into the hint for the Silver Arrows, which can make a difference as well.
Roll the dice!
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White Sword OR Ladder Heart Shuffled
White Sword AND Ladder Heart NOT Shuffled
White Sword AND Ladder Heart Shuffled
Special thanks to Kababesh for helping fix errors and typos!