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 $20.88   10.587%   $23.0906   $23.09   $23.10   $23.10  
  Purchase #2 $7.45   6.401%   $7.9269   $7.93   $7.95   $7.90  
  Purchase #3 $138.69   12.645%   $156.2274   $156.23   $156.25   $156.20  
  Purchase #4 $74.89   10.197%   $82.5265   $82.53   $82.55   $82.50  
  Purchase #5 $66.58   5.373%   $70.1573   $70.16   $70.15   $70.20  
  Purchase #6 $11.59   7.335%   $12.4401   $12.44   $12.45   $12.40  
  Purchase #7 $87.29   12.585%   $98.2754   $98.28   $98.30   $98.30  
  7 purchase totals = $407.37       $450.6442   $450.66   $450.75   $450.60