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 $234.98   6.607%   $250.5051   $250.51   $250.50   $250.50  
  Purchase #2 $21.29   12.336%   $23.9163   $23.92   $23.90   $23.90  
  Purchase #3 $102.85   5.579%   $108.588   $108.59   $108.60   $108.60  
  Purchase #4 $60.95   8.166%   $65.9272   $65.93   $65.95   $65.90  
  Purchase #5 $228.85   12.361%   $257.1381   $257.14   $257.15   $257.10  
  Purchase #6 $247.94   3.317%   $256.1642   $256.16   $256.15   $256.20  
  Purchase #7 $64.48   4.540%   $67.4074   $67.41   $67.40   $67.40  
  7 purchase totals = $961.34       $1,029.6463   $1,029.66   $1,029.65   $1,029.60