Easy question..Please help!!

 
Hi, Basic question here... If I have a BUY order (lets say EURUSD) to be placed at 1.5800....Should I place it when Bid reaches 1.5800 or when Buy reaches 1.5800?? If I wait for Ask price...I may be missing some pips (the spread)... For which price should an EA wait for?? Thanks in advance,
 
BUY order opening at Ask only.
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