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 $17.63   6.670%   $18.8059   $18.81   $18.80   $18.80  
  Purchase #2 $194.31   5.115%   $204.249   $204.25   $204.25   $204.20  
  Purchase #3 $9.89   9.195%   $10.7994   $10.80   $10.80   $10.80  
  Purchase #4 $31.97   7.156%   $34.2578   $34.26   $34.25   $34.30  
  Purchase #5 $77.80   7.630%   $83.7361   $83.74   $83.75   $83.70  
  Purchase #6 $43.96   9.436%   $48.1081   $48.11   $48.10   $48.10  
  Purchase #7 $229.99   5.700%   $243.0994   $243.10   $243.10   $243.10  
  7 purchase totals = $605.55       $643.0557   $643.07   $643.05   $643.00