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 $231.44   12.309%   $259.9279   $259.93   $259.95   $259.90  
  Purchase #2 $243.72   11.866%   $272.6398   $272.64   $272.65   $272.60  
  Purchase #3 $215.73   9.107%   $235.3765   $235.38   $235.40   $235.40  
  Purchase #4 $67.57   7.476%   $72.6215   $72.62   $72.60   $72.60  
  Purchase #5 $205.87   10.715%   $227.929   $227.93   $227.95   $227.90  
  Purchase #6 $230.55   4.103%   $240.0095   $240.01   $240.00   $240.00  
  Purchase #7 $116.71   8.556%   $126.6957   $126.70   $126.70   $126.70  
  7 purchase totals = $1,311.59       $1,435.1999   $1,435.21   $1,435.25   $1,435.10