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 $81.31   3.861%   $84.4494   $84.45   $84.45   $84.40  
  Purchase #2 $67.32   12.799%   $75.9363   $75.94   $75.95   $75.90  
  Purchase #3 $91.51   5.517%   $96.5586   $96.56   $96.55   $96.60  
  Purchase #4 $7.38   10.833%   $8.1795   $8.18   $8.20   $8.20  
  Purchase #5 $208.32   12.857%   $235.1037   $235.10   $235.10   $235.10  
  Purchase #6 $16.51   4.860%   $17.3124   $17.31   $17.30   $17.30  
  Purchase #7 $59.81   4.112%   $62.2694   $62.27   $62.25   $62.30  
  7 purchase totals = $532.16       $579.8093   $579.81   $579.80   $579.80