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 $9.75   6.688%   $10.4021   $10.40   $10.40   $10.40  
  Purchase #2 $28.28   12.550%   $31.8291   $31.83   $31.85   $31.80  
  Purchase #3 $133.96   12.789%   $151.0921   $151.09   $151.10   $151.10  
  Purchase #4 $88.92   10.328%   $98.1037   $98.10   $98.10   $98.10  
  Purchase #5 $174.92   4.799%   $183.3144   $183.31   $183.30   $183.30  
  Purchase #6 $91.70   8.576%   $99.5642   $99.56   $99.55   $99.60  
  Purchase #7 $151.53   3.663%   $157.0805   $157.08   $157.10   $157.10  
  7 purchase totals = $679.06       $731.3861   $731.37   $731.40   $731.40