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 $246.59   11.863%   $275.843   $275.84   $275.85   $275.80  
  Purchase #2 $65.37   7.763%   $70.4447   $70.44   $70.45   $70.40  
  Purchase #3 $250.37   7.418%   $268.9424   $268.94   $268.95   $268.90  
  Purchase #4 $119.27   10.212%   $131.4499   $131.45   $131.45   $131.40  
  Purchase #5 $6.38   3.505%   $6.6036   $6.60   $6.60   $6.60  
  Purchase #6 $129.89   7.584%   $139.7409   $139.74   $139.75   $139.70  
  Purchase #7 $71.88   7.535%   $77.2962   $77.30   $77.30   $77.30  
  7 purchase totals = $889.75       $970.3207   $970.31   $970.35   $970.10