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 $226.85   9.774%   $249.0223   $249.02   $249.00   $249.00  
  Purchase #2 $216.40   12.504%   $243.4587   $243.46   $243.45   $243.50  
  Purchase #3 $244.26   12.264%   $274.216   $274.22   $274.20   $274.20  
  Purchase #4 $167.89   5.193%   $176.6085   $176.61   $176.60   $176.60  
  Purchase #5 $30.62   8.444%   $33.2056   $33.21   $33.20   $33.20  
  Purchase #6 $69.66   4.254%   $72.6233   $72.62   $72.60   $72.60  
  Purchase #7 $91.38   11.338%   $101.7407   $101.74   $101.75   $101.70  
  7 purchase totals = $1,047.06       $1,150.8751   $1,150.88   $1,150.80   $1,150.80