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 $81.35   8.325%   $88.1224   $88.12   $88.10   $88.10  
  Purchase #2 $125.41   6.871%   $134.0269   $134.03   $134.05   $134.00  
  Purchase #3 $40.84   4.165%   $42.541   $42.54   $42.55   $42.50  
  Purchase #4 $105.73   9.362%   $115.6284   $115.63   $115.65   $115.60  
  Purchase #5 $29.42   6.314%   $31.2776   $31.28   $31.30   $31.30  
  Purchase #6 $144.28   7.488%   $155.0837   $155.08   $155.10   $155.10  
  Purchase #7 $70.47   4.523%   $73.6574   $73.66   $73.65   $73.70  
  7 purchase totals = $597.50       $640.3374   $640.34   $640.40   $640.30