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 $216.58   12.405%   $243.4467   $243.45   $243.45   $243.40  
  Purchase #2 $170.96   13.747%   $194.4619   $194.46   $194.45   $194.50  
  Purchase #3 $131.96   12.463%   $148.4062   $148.41   $148.40   $148.40  
  Purchase #4 $236.39   13.641%   $268.636   $268.64   $268.65   $268.60  
  Purchase #5 $125.81   13.713%   $143.0623   $143.06   $143.05   $143.10  
  Purchase #6 $186.91   5.425%   $197.0499   $197.05   $197.05   $197.00  
  Purchase #7 $112.36   12.801%   $126.7432   $126.74   $126.75   $126.70  
  7 purchase totals = $1,180.97       $1,321.8062   $1,321.81   $1,321.80   $1,321.70