[minminRW]

In this case おく means preperation of going and indicates the decision in short.

Then 行っておけばよかった is used when the speaker makes a point on the past dicision.
That affection is not saying directly, but include the meaning "I must have made a decision to do in advance."

For example, If people who have gone got special gifts, both sentences are OK.
「プレゼントがもらえるなら、行けばよかった」He regrets he did not go.
「プレゼントがもらえるなら、行っとけばよかった」 He regrets his past decision of not going.

If the speaker had another plan but he finally failed, 行っとけばよかった fits rather than 行けばよかった.
「デートはやめたので、行っとけばよかった」

If the people who did not go were forced to do homework,
「宿題をするくらいなら、行っとけばよかった。」 is more natural.

These feelings are very fine. In all case I wrote, both sentences could be use.