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 $18.44   9.456%   $20.1837   $20.18   $20.20   $20.20  
  Purchase #2 $84.49   10.669%   $93.5042   $93.50   $93.50   $93.50  
  Purchase #3 $134.83   13.612%   $153.1831   $153.18   $153.20   $153.20  
  Purchase #4 $31.88   6.470%   $33.9426   $33.94   $33.95   $33.90  
  Purchase #5 $242.73   10.657%   $268.5977   $268.60   $268.60   $268.60  
  Purchase #6 $11.97   9.306%   $13.0839   $13.08   $13.10   $13.10  
  Purchase #7 $145.54   10.156%   $160.321   $160.32   $160.30   $160.30  
  7 purchase totals = $669.88       $742.8162   $742.80   $742.85   $742.80