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 $6.55   3.367%   $6.7705   $6.77   $6.75   $6.80  
  Purchase #2 $18.87   11.189%   $20.9814   $20.98   $21.00   $21.00  
  Purchase #3 $227.62   9.438%   $249.1028   $249.10   $249.10   $249.10  
  Purchase #4 $93.57   10.138%   $103.0561   $103.06   $103.05   $103.10  
  Purchase #5 $153.48   4.498%   $160.3835   $160.38   $160.40   $160.40  
  Purchase #6 $116.97   13.398%   $132.6416   $132.64   $132.65   $132.60  
  Purchase #7 $147.42   13.776%   $167.7286   $167.73   $167.75   $167.70  
  7 purchase totals = $764.48       $840.6645   $840.66   $840.70   $840.70