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 $231.59   6.665%   $247.0255   $247.03   $247.05   $247.00  
  Purchase #2 $164.39   9.148%   $179.4284   $179.43   $179.45   $179.40  
  Purchase #3 $139.46   8.268%   $150.9906   $150.99   $151.00   $151.00  
  Purchase #4 $95.25   10.311%   $105.0712   $105.07   $105.05   $105.10  
  Purchase #5 $13.89   11.564%   $15.4962   $15.50   $15.50   $15.50  
  Purchase #6 $120.80   9.479%   $132.2506   $132.25   $132.25   $132.30  
  Purchase #7 $46.80   8.334%   $50.7003   $50.70   $50.70   $50.70  
  7 purchase totals = $812.18       $880.9628   $880.97   $881.00   $881.00