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 $20.36   7.582%   $21.9037   $21.90   $21.90   $21.90  
  Purchase #2 $54.45   12.716%   $61.3739   $61.37   $61.35   $61.40  
  Purchase #3 $33.97   3.684%   $35.2215   $35.22   $35.20   $35.20  
  Purchase #4 $121.32   7.710%   $130.6738   $130.67   $130.65   $130.70  
  Purchase #5 $77.82   4.422%   $81.2612   $81.26   $81.25   $81.30  
  Purchase #6 $74.69   8.499%   $81.0379   $81.04   $81.05   $81.00  
  Purchase #7 $55.76   6.497%   $59.3827   $59.38   $59.40   $59.40  
  7 purchase totals = $438.37       $470.8547   $470.84   $470.80   $470.90