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 $87.69   3.559%   $90.8109   $90.81   $90.80   $90.80  
  Purchase #2 $75.90   10.377%   $83.7761   $83.78   $83.80   $83.80  
  Purchase #3 $206.46   7.192%   $221.3086   $221.31   $221.30   $221.30  
  Purchase #4 $98.29   12.155%   $110.2371   $110.24   $110.25   $110.20  
  Purchase #5 $233.92   7.559%   $251.602   $251.60   $251.60   $251.60  
  Purchase #6 $193.75   5.217%   $203.8579   $203.86   $203.85   $203.90  
  Purchase #7 $236.72   10.223%   $260.9199   $260.92   $260.90   $260.90  
  7 purchase totals = $1,132.73       $1,222.5125   $1,222.52   $1,222.50   $1,222.50