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 $28.74   4.250%   $29.9615   $29.96   $29.95   $30.00  
  Purchase #2 $219.35   5.359%   $231.105   $231.11   $231.10   $231.10  
  Purchase #3 $154.62   10.235%   $170.4454   $170.45   $170.45   $170.40  
  Purchase #4 $215.77   10.819%   $239.1142   $239.11   $239.10   $239.10  
  Purchase #5 $157.55   13.801%   $179.2935   $179.29   $179.30   $179.30  
  Purchase #6 $6.40   11.516%   $7.137   $7.14   $7.15   $7.10  
  Purchase #7 $110.61   4.622%   $115.7224   $115.72   $115.70   $115.70  
  7 purchase totals = $893.04       $972.7790   $972.78   $972.75   $972.70