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 $70.77   13.762%   $80.5094   $80.51   $80.50   $80.50  
  Purchase #2 $217.37   12.154%   $243.7891   $243.79   $243.80   $243.80  
  Purchase #3 $232.96   3.757%   $241.7123   $241.71   $241.70   $241.70  
  Purchase #4 $199.85   8.456%   $216.7493   $216.75   $216.75   $216.70  
  Purchase #5 $65.29   10.429%   $72.0991   $72.10   $72.10   $72.10  
  Purchase #6 $28.84   8.419%   $31.268   $31.27   $31.25   $31.30  
  Purchase #7 $44.34   12.589%   $49.922   $49.92   $49.90   $49.90  
  7 purchase totals = $859.42       $936.0492   $936.05   $936.00   $936.00