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 $250.92   6.201%   $266.4795   $266.48   $266.50   $266.50  
  Purchase #2 $167.48   8.428%   $181.5952   $181.60   $181.60   $181.60  
  Purchase #3 $37.42   11.216%   $41.617   $41.62   $41.60   $41.60  
  Purchase #4 $52.57   4.320%   $54.841   $54.84   $54.85   $54.80  
  Purchase #5 $108.39   10.120%   $119.3591   $119.36   $119.35   $119.40  
  Purchase #6 $64.47   3.117%   $66.4795   $66.48   $66.50   $66.50  
  Purchase #7 $117.59   12.845%   $132.6944   $132.69   $132.70   $132.70  
  7 purchase totals = $798.84       $863.0657   $863.07   $863.10   $863.10