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 $157.83   9.475%   $172.7844   $172.78   $172.80   $172.80  
  Purchase #2 $78.31   7.858%   $84.4636   $84.46   $84.45   $84.50  
  Purchase #3 $97.59   6.288%   $103.7265   $103.73   $103.75   $103.70  
  Purchase #4 $62.97   6.665%   $67.167   $67.17   $67.15   $67.20  
  Purchase #5 $204.78   11.760%   $228.8621   $228.86   $228.85   $228.90  
  Purchase #6 $40.52   10.768%   $44.8832   $44.88   $44.90   $44.90  
  Purchase #7 $17.75   11.571%   $19.8039   $19.80   $19.80   $19.80  
  7 purchase totals = $659.75       $721.6907   $721.68   $721.70   $721.80