Unintentional sideslip (flanking and turning) have no modifiers on the PSR. Even damage in this case, as the critical damage that would apply modifiers (stabilizer) also restricts the unit from using flank movement, meaning you can't put yourself in the position to sideslip. You do, however, have to make the roll any time you change facing and move forward a hex at flank. This means that potentially you'd roll for a sideslip several times in one movement if you get real squirrely with it. (pg 67-68, Total Warfare, as you quoted initially). The only additional modifier I can think of is for the use of Sprinting/Overdrive movement, which would convey a +2 on each PSR (pg. 16, TO: AR).
Controlled sideslips (pg. 23, TO: AR), have a -1 modifier on the PSR.