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 $71.92   6.449%   $76.5581   $76.56   $76.55   $76.60  
  Purchase #2 $115.41   3.108%   $118.9969   $119.00   $119.00   $119.00  
  Purchase #3 $238.72   4.355%   $249.1163   $249.12   $249.10   $249.10  
  Purchase #4 $30.79   7.119%   $32.9819   $32.98   $33.00   $33.00  
  Purchase #5 $132.55   13.364%   $150.264   $150.26   $150.25   $150.30  
  Purchase #6 $78.44   9.119%   $85.5929   $85.59   $85.60   $85.60  
  Purchase #7 $146.53   12.756%   $165.2214   $165.22   $165.20   $165.20  
  7 purchase totals = $814.36       $878.7315   $878.73   $878.70   $878.80