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 $18.69   11.857%   $20.9061   $20.91   $20.90   $20.90  
  Purchase #2 $77.45   6.810%   $82.7243   $82.72   $82.70   $82.70  
  Purchase #3 $31.50   5.649%   $33.2794   $33.28   $33.30   $33.30  
  Purchase #4 $11.45   8.139%   $12.3819   $12.38   $12.40   $12.40  
  Purchase #5 $203.44   12.397%   $228.6605   $228.66   $228.65   $228.70  
  Purchase #6 $119.40   8.218%   $129.2123   $129.21   $129.20   $129.20  
  Purchase #7 $104.72   12.586%   $117.9001   $117.90   $117.90   $117.90  
  7 purchase totals = $566.65       $625.0646   $625.06   $625.05   $625.10