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 $140.49   7.370%   $150.8441   $150.84   $150.85   $150.80  
  Purchase #2 $7.68   5.611%   $8.1109   $8.11   $8.10   $8.10  
  Purchase #3 $80.57   11.308%   $89.6809   $89.68   $89.70   $89.70  
  Purchase #4 $19.90   9.328%   $21.7563   $21.76   $21.75   $21.80  
  Purchase #5 $40.45   11.722%   $45.1915   $45.19   $45.20   $45.20  
  Purchase #6 $81.81   3.448%   $84.6308   $84.63   $84.65   $84.60  
  Purchase #7 $134.87   11.444%   $150.3045   $150.30   $150.30   $150.30  
  7 purchase totals = $505.77       $550.5190   $550.51   $550.55   $550.50