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 $9.99   3.354%   $10.3251   $10.33   $10.35   $10.30  
  Purchase #2 $98.27   4.656%   $102.8455   $102.85   $102.85   $102.80  
  Purchase #3 $94.38   13.865%   $107.4658   $107.47   $107.45   $107.50  
  Purchase #4 $51.79   13.776%   $58.9246   $58.92   $58.90   $58.90  
  Purchase #5 $39.50   11.669%   $44.1093   $44.11   $44.10   $44.10  
  Purchase #6 $71.71   7.360%   $76.9879   $76.99   $77.00   $77.00  
  Purchase #7 $144.50   7.190%   $154.8896   $154.89   $154.90   $154.90  
  7 purchase totals = $510.14       $555.5478   $555.56   $555.55   $555.50