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 $137.80   4.309%   $143.7378   $143.74   $143.75   $143.70  
  Purchase #2 $78.60   6.648%   $83.8253   $83.83   $83.85   $83.80  
  Purchase #3 $9.32   5.450%   $9.8279   $9.83   $9.85   $9.80  
  Purchase #4 $49.50   10.662%   $54.7777   $54.78   $54.80   $54.80  
  Purchase #5 $250.72   12.676%   $282.5013   $282.50   $282.50   $282.50  
  Purchase #6 $185.92   12.520%   $209.1972   $209.20   $209.20   $209.20  
  Purchase #7 $93.28   11.865%   $104.3477   $104.35   $104.35   $104.30  
  7 purchase totals = $805.14       $888.2149   $888.23   $888.30   $888.10