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 $16.65   10.602%   $18.4152   $18.42   $18.40   $18.40  
  Purchase #2 $37.33   8.767%   $40.6027   $40.60   $40.60   $40.60  
  Purchase #3 $11.55   5.256%   $12.1571   $12.16   $12.15   $12.20  
  Purchase #4 $108.73   3.284%   $112.3007   $112.30   $112.30   $112.30  
  Purchase #5 $29.40   9.537%   $32.2039   $32.20   $32.20   $32.20  
  Purchase #6 $104.87   10.723%   $116.1152   $116.12   $116.10   $116.10  
  Purchase #7 $177.80   11.658%   $198.5279   $198.53   $198.55   $198.50  
  7 purchase totals = $486.33       $530.3227   $530.33   $530.30   $530.30