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 $39.80   12.423%   $44.7444   $44.74   $44.75   $44.70  
  Purchase #2 $113.62   9.351%   $124.2446   $124.24   $124.25   $124.20  
  Purchase #3 $148.67   7.279%   $159.4917   $159.49   $159.50   $159.50  
  Purchase #4 $238.26   6.844%   $254.5665   $254.57   $254.55   $254.60  
  Purchase #5 $161.26   6.158%   $171.1904   $171.19   $171.20   $171.20  
  Purchase #6 $129.54   12.236%   $145.3905   $145.39   $145.40   $145.40  
  Purchase #7 $65.82   6.863%   $70.3372   $70.34   $70.35   $70.30  
  7 purchase totals = $896.97       $969.9653   $969.96   $970.00   $969.90