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 $18.92   6.413%   $20.1333   $20.13   $20.15   $20.10  
  Purchase #2 $26.90   10.667%   $29.7694   $29.77   $29.75   $29.80  
  Purchase #3 $25.43   3.145%   $26.2298   $26.23   $26.25   $26.20  
  Purchase #4 $182.40   4.258%   $190.1666   $190.17   $190.15   $190.20  
  Purchase #5 $185.94   7.607%   $200.0845   $200.08   $200.10   $200.10  
  Purchase #6 $28.95   8.224%   $31.3308   $31.33   $31.35   $31.30  
  Purchase #7 $80.63   11.627%   $90.0049   $90.00   $90.00   $90.00  
  7 purchase totals = $549.17       $587.7193   $587.71   $587.75   $587.70