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 $9.67   7.807%   $10.4249   $10.42   $10.40   $10.40  
  Purchase #2 $93.72   6.582%   $99.8887   $99.89   $99.90   $99.90  
  Purchase #3 $17.60   11.745%   $19.6671   $19.67   $19.65   $19.70  
  Purchase #4 $37.99   12.411%   $42.7049   $42.70   $42.70   $42.70  
  Purchase #5 $137.82   4.344%   $143.8069   $143.81   $143.80   $143.80  
  Purchase #6 $8.65   7.774%   $9.3225   $9.32   $9.30   $9.30  
  Purchase #7 $17.46   10.836%   $19.352   $19.35   $19.35   $19.40  
  7 purchase totals = $322.91       $345.1670   $345.16   $345.10   $345.20