Cullen steered the unbeaten Guinness PRO14 leaders to a 43-16 win at Franklin's Gardens on Saturday.
It meant they took control at the top of Champions Cup Pool 1, denting Saints' hopes of progression in the process.
It was the third time in as many matches that Leinster had won the Northampton leg of a Champions Cup double-header.
They also did so in 2013 and 2016, but in 2013 Leinster suffered a shock on home soil as Saints bounced back from a humbling 40-7 defeat by winning 18-9 in Dublin on the following weekend.
And Cullen said: "We've been in this situation before where we've had a big win in Northampton and gone back to the Aviva been beaten.
"You've to be so careful with these fixtures. Normally, it's just onto the next team. It's a unique thing that takes place in the season where there's head-to-head fixtures. It doesn't always transfer into the following week.
"We've seen that first-hand ourselves in this exact fixture where we've had a win - similar to what we had on Saturday - and then go back to the Aviva the following week and Northampton were very clever that day.
"They kicked us into the corner, had a lot of possession, George North eventually scored. They put a lot of pressure on us and we just literally ran out of time.
"At the end of the game, we were pushing hard, dropped a ball, they ran the length of the field and we lost out on even a losing bonus point.
"That meant we were away in the quarter-final against Toulon which I remember because it was my last game as a player in this competition.
"There's a huge knock-on effect of every single point in these round robin games. We can't take anything for granted because every point is so important."
Leinster scored seven tries at Saints on Saturday, coming back from 16-14 down and completely shutting out Chris Boyd's team in the second half.
"We looked stretched at different stages, particularly in the first half and even for bits of the second half," Cullen said.
"Northampton had us a couple of times where they don't quite nail that final pass. So, there's lots for us to improve on and it's important we have that mindset.
"Leading into the game, we've talked about not quite taking chances that have been there for us in games. Even in the first half, I thought we pushed things a little bit.
"We tightened up a lot better in the second half, we'd much better ball control and two good tries early in the second half and two good tries at the end and that was the most pleasing piece."
One potential blow for Leinster ahead of next weekend's game was the injury suffered by Ireland fly-half Jonny Sexton.
"Johnny has taken a bang to the knee and we'll see how much damage has been done," Cullen said.
"We're just trying to build cohesion with the guys coming back from the World Cup and that's taking a bit of time.
"This was a positive step, but we'll get better each week and continue to improve."