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 $238.81   4.521%   $249.6066   $249.61   $249.60   $249.60  
  Purchase #2 $111.59   12.224%   $125.2308   $125.23   $125.25   $125.20  
  Purchase #3 $37.52   9.574%   $41.1122   $41.11   $41.10   $41.10  
  Purchase #4 $87.33   6.682%   $93.1654   $93.17   $93.15   $93.20  
  Purchase #5 $148.60   8.842%   $161.7392   $161.74   $161.75   $161.70  
  Purchase #6 $138.79   7.166%   $148.7357   $148.74   $148.75   $148.70  
  Purchase #7 $31.52   8.328%   $34.145   $34.15   $34.15   $34.10  
  7 purchase totals = $794.16       $853.7349   $853.75   $853.75   $853.60