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 $235.67   13.113%   $266.5734   $266.57   $266.55   $266.60  
  Purchase #2 $112.71   7.111%   $120.7248   $120.72   $120.70   $120.70  
  Purchase #3 $26.30   11.409%   $29.3006   $29.30   $29.30   $29.30  
  Purchase #4 $203.48   10.743%   $225.3399   $225.34   $225.35   $225.30  
  Purchase #5 $152.32   7.383%   $163.5658   $163.57   $163.55   $163.60  
  Purchase #6 $137.35   6.675%   $146.5181   $146.52   $146.50   $146.50  
  Purchase #7 $126.32   4.782%   $132.3606   $132.36   $132.35   $132.40  
  7 purchase totals = $994.15       $1,084.3832   $1,084.38   $1,084.30   $1,084.40