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 $121.82   11.629%   $135.9864   $135.99   $136.00   $136.00  
  Purchase #2 $217.64   5.332%   $229.2446   $229.24   $229.25   $229.20  
  Purchase #3 $178.83   7.577%   $192.3799   $192.38   $192.40   $192.40  
  Purchase #4 $130.63   13.795%   $148.6504   $148.65   $148.65   $148.70  
  Purchase #5 $212.35   8.230%   $229.8264   $229.83   $229.85   $229.80  
  Purchase #6 $222.40   12.701%   $250.647   $250.65   $250.65   $250.60  
  Purchase #7 $243.85   12.294%   $273.8289   $273.83   $273.85   $273.80  
  7 purchase totals = $1,327.52       $1,460.5636   $1,460.57   $1,460.65   $1,460.50