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 $16.52   13.720%   $18.7865   $18.79   $18.80   $18.80  
  Purchase #2 $120.93   12.600%   $136.1672   $136.17   $136.15   $136.20  
  Purchase #3 $164.60   3.792%   $170.8416   $170.84   $170.85   $170.80  
  Purchase #4 $177.73   5.195%   $186.9631   $186.96   $186.95   $187.00  
  Purchase #5 $243.63   11.711%   $272.1615   $272.16   $272.15   $272.20  
  Purchase #6 $241.29   8.451%   $261.6814   $261.68   $261.70   $261.70  
  Purchase #7 $78.57   5.371%   $82.79   $82.79   $82.80   $82.80  
  7 purchase totals = $1,043.27       $1,129.3913   $1,129.39   $1,129.40   $1,129.50