BlackJack Basic Strategy
Wikipedia's article for basic strategy says to stand when dealt a hard 18-21, at all times, regardless of the dealer's face up card.

Given a game with the following parameters:


Four to eight decks
The dealer hits on a soft 17
A double is allowed after a split
Only original bets are lost on dealer blackjack






https://en.wikipedia.org/wiki/Blackjack#Basic_strategy
Source:  "4-Deck to 8-Deck Blackjack Strategy - Wizard of Odds". Wizard of Odds Consulting, Inc. Retrieved April 30, 2014. http://wizardofodds.com/games/blackjack/strategy/4-decks/