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 $26.53   11.136%   $29.4844   $29.48   $29.50   $29.50  
  Purchase #2 $59.32   8.259%   $64.2192   $64.22   $64.20   $64.20  
  Purchase #3 $73.86   13.777%   $84.0357   $84.04   $84.05   $84.00  
  Purchase #4 $181.85   3.166%   $187.6074   $187.61   $187.60   $187.60  
  Purchase #5 $223.72   8.674%   $243.1255   $243.13   $243.15   $243.10  
  Purchase #6 $161.84   8.256%   $175.2015   $175.20   $175.20   $175.20  
  Purchase #7 $223.62   5.778%   $236.5408   $236.54   $236.55   $236.50  
  7 purchase totals = $950.74       $1,020.2145   $1,020.22   $1,020.25   $1,020.10