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 $222.59   12.306%   $249.9819   $249.98   $250.00   $250.00  
  Purchase #2 $115.78   8.831%   $126.0045   $126.00   $126.00   $126.00  
  Purchase #3 $109.40   7.554%   $117.6641   $117.66   $117.65   $117.70  
  Purchase #4 $203.50   9.406%   $222.6412   $222.64   $222.65   $222.60  
  Purchase #5 $202.51   5.405%   $213.4557   $213.46   $213.45   $213.50  
  Purchase #6 $165.51   8.767%   $180.0203   $180.02   $180.00   $180.00  
  Purchase #7 $248.31   10.257%   $273.7792   $273.78   $273.80   $273.80  
  7 purchase totals = $1,267.60       $1,383.5469   $1,383.54   $1,383.55   $1,383.60