Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $238.72   9.194%   $260.6679   $260.67   $260.65   $260.70  
  Purchase #2 $169.83   13.592%   $192.9133   $192.91   $192.90   $192.90  
  Purchase #3 $81.80   11.669%   $91.3452   $91.35   $91.35   $91.30  
  Purchase #4 $210.87   5.654%   $222.7926   $222.79   $222.80   $222.80  
  Purchase #5 $102.60   8.723%   $111.5498   $111.55   $111.55   $111.50  
  Purchase #6 $17.54   6.515%   $18.6827   $18.68   $18.70   $18.70  
  Purchase #7 $126.73   4.336%   $132.225   $132.23   $132.25   $132.20  
  7 purchase totals = $948.09       $1,030.1765   $1,030.18   $1,030.20   $1,030.10