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 $25.68   12.370%   $28.8566   $28.86   $28.85   $28.90  
  Purchase #2 $7.86   12.237%   $8.8218   $8.82   $8.80   $8.80  
  Purchase #3 $29.53   3.588%   $30.5895   $30.59   $30.60   $30.60  
  Purchase #4 $190.62   13.783%   $216.8932   $216.89   $216.90   $216.90  
  Purchase #5 $26.97   8.236%   $29.1912   $29.19   $29.20   $29.20  
  Purchase #6 $86.43   11.597%   $96.4533   $96.45   $96.45   $96.50  
  Purchase #7 $38.36   4.747%   $40.1809   $40.18   $40.20   $40.20  
  7 purchase totals = $405.45       $450.9865   $450.98   $451.00   $451.10