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 $163.90   9.463%   $179.4099   $179.41   $179.40   $179.40  
  Purchase #2 $91.47   10.562%   $101.1311   $101.13   $101.15   $101.10  
  Purchase #3 $37.73   3.279%   $38.9672   $38.97   $38.95   $39.00  
  Purchase #4 $213.71   11.646%   $238.5987   $238.60   $238.60   $238.60  
  Purchase #5 $6.78   13.382%   $7.6873   $7.69   $7.70   $7.70  
  Purchase #6 $55.35   12.464%   $62.2488   $62.25   $62.25   $62.20  
  Purchase #7 $241.92   3.639%   $250.7235   $250.72   $250.70   $250.70  
  7 purchase totals = $810.86       $878.7665   $878.77   $878.75   $878.70