in my opinion it depends on whether it is a long or short term...

in my opinion it depends on whether it is a long or short term trade

if it is a day trade and you think it will fly, the open may be the lowest price ever. but first check how much the stock has already moved over the last month or so, because if it has moved a good distance a rally will often be sold into

if it is a swing trade of a blue chip or mid cap then if the stock is gapping up a lot in the morning could be a good idea to wait for the afternoon for a better price

if you have access to VWAP then try to buy close to that price