Playing dirty = getting opponents to hit traffic, walls, etc.

I've done it once, rammed an opponent right into a HiAce in TXR3 and zoomed off while they were recovering.

Well i was playing dirty too in TXRD2.. Ramming my rivals in the race

LOL it's remind me with Maximum Tune

I usually don't, but sometimes I just can't do otherwise, especially on a 2-line road with one traffic car in front of my rival, i just stay close until he brakes!

but I usually try racing clean though

It's almost impossible to win sometimes in TXRD2 playing completely clean. tracks are so narrow and hard to pass. especially if you are driving underpowered cars that rely on cornering advantages. and it's frustrating how computer seems to corner so slow but block roads, and always seem to make me spin the cheap way when following. I often end up hitting walls because of trying to find little gaps.

TXR series, I do it against fast opponents since it's the easiest way to win. Easy opponents, I race cleanly to save CP bars for more money.

all the time...

I also playing dirty on TXRD2 , sometime i play clean. Its really hard to pass clean

It depends on the situation.

I try to play clean but if the CPU starts ramming me, he's dead

I do what has to be done, haven't gotten TXRD2 yet but in ITC and TXR3 I have pushed them aside a few times thus far.
But most of the time I find the opponent is either too easy (thus you win by a longshot) or too hard (you get blown out) making it so that it ends up I drive cleanly anyways.

If I beat the rival off the ine, no.
Otherwise, always.