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 $243.50   13.178%   $275.5884   $275.59   $275.60   $275.60  
  Purchase #2 $211.62   12.665%   $238.4217   $238.42   $238.40   $238.40  
  Purchase #3 $227.67   11.809%   $254.5556   $254.56   $254.55   $254.60  
  Purchase #4 $27.94   8.337%   $30.2694   $30.27   $30.25   $30.30  
  Purchase #5 $220.38   3.498%   $228.0889   $228.09   $228.10   $228.10  
  Purchase #6 $204.25   9.483%   $223.619   $223.62   $223.60   $223.60  
  Purchase #7 $59.30   5.528%   $62.5781   $62.58   $62.60   $62.60  
  7 purchase totals = $1,194.66       $1,313.1211   $1,313.13   $1,313.10   $1,313.20