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 $193.99   11.779%   $216.8401   $216.84   $216.85   $216.80  
  Purchase #2 $225.90   7.115%   $241.9728   $241.97   $241.95   $242.00  
  Purchase #3 $29.99   9.783%   $32.9239   $32.92   $32.90   $32.90  
  Purchase #4 $106.53   4.181%   $110.984   $110.98   $111.00   $111.00  
  Purchase #5 $84.32   5.298%   $88.7873   $88.79   $88.80   $88.80  
  Purchase #6 $36.49   11.438%   $40.6637   $40.66   $40.65   $40.70  
  Purchase #7 $236.46   13.762%   $269.0016   $269.00   $269.00   $269.00  
  7 purchase totals = $913.68       $1,001.1734   $1,001.16   $1,001.15   $1,001.20