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 $6.70   4.458%   $6.9987   $7.00   $7.00   $7.00  
  Purchase #2 $19.96   6.339%   $21.2253   $21.23   $21.25   $21.20  
  Purchase #3 $219.67   4.142%   $228.7687   $228.77   $228.75   $228.80  
  Purchase #4 $18.67   9.532%   $20.4496   $20.45   $20.45   $20.40  
  Purchase #5 $245.46   6.394%   $261.1547   $261.15   $261.15   $261.20  
  Purchase #6 $73.52   12.594%   $82.7791   $82.78   $82.80   $82.80  
  Purchase #7 $169.85   10.150%   $187.0898   $187.09   $187.10   $187.10  
  7 purchase totals = $753.83       $808.4659   $808.47   $808.50   $808.50