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 $249.50   9.801%   $273.9535   $273.95   $273.95   $274.00  
  Purchase #2 $148.62   8.610%   $161.4162   $161.42   $161.40   $161.40  
  Purchase #3 $76.99   9.107%   $84.0015   $84.00   $84.00   $84.00  
  Purchase #4 $23.95   6.627%   $25.5372   $25.54   $25.55   $25.50  
  Purchase #5 $221.91   11.323%   $247.0369   $247.04   $247.05   $247.00  
  Purchase #6 $146.81   12.338%   $164.9234   $164.92   $164.90   $164.90  
  Purchase #7 $126.35   7.275%   $135.542   $135.54   $135.55   $135.50  
  7 purchase totals = $994.13       $1,092.4107   $1,092.41   $1,092.40   $1,092.30