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 $28.64   7.815%   $30.8782   $30.88   $30.90   $30.90  
  Purchase #2 $146.63   6.142%   $155.636   $155.64   $155.65   $155.60  
  Purchase #3 $77.57   3.816%   $80.5301   $80.53   $80.55   $80.50  
  Purchase #4 $21.97   12.134%   $24.6358   $24.64   $24.65   $24.60  
  Purchase #5 $126.47   8.411%   $137.1074   $137.11   $137.10   $137.10  
  Purchase #6 $9.31   13.654%   $10.5812   $10.58   $10.60   $10.60  
  Purchase #7 $67.36   6.268%   $71.5821   $71.58   $71.60   $71.60  
  7 purchase totals = $477.95       $510.9508   $510.96   $511.05   $510.90