There is absolutely no point to open parlays. The only time you need to put in a parlay is if two games are being played at the same time. Otherwise, a parlay is exactly the same as "letting it ride" on your next bet. (In fact, except for 3-teamers, it is slightly worse to parlay than it is to just let the winnnings ride).
Example: you put in a four team "open parlay", which pays you 12:1 (at pinnacle, lot of books have it at 10:1, which is a rip-off). You risk 10 to win 120. Meanwhile, I bet the same games one at a time and "let it ride". Here is the bets I would make:
Game 1: risk 10 to win 9.09 (total = 19.09)
Game 2: risk 19.09 to win 17.35 (total = 36.44)
Game 3: risk 36.44 to win 33.13 (total = 69.57)
Game 4: risk 69.57 to win 63.25 (total = 132.82, my original $10 plus 122.82 in winnings)
In reality, i actually made slightly more money doing it manually than by "parlaying" it with the book.