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 $86.86   13.219%   $98.342   $98.34   $98.35   $98.30  
  Purchase #2 $84.25   5.323%   $88.7346   $88.73   $88.75   $88.70  
  Purchase #3 $91.53   5.445%   $96.5138   $96.51   $96.50   $96.50  
  Purchase #4 $80.46   5.425%   $84.825   $84.83   $84.85   $84.80  
  Purchase #5 $148.79   4.255%   $155.121   $155.12   $155.10   $155.10  
  Purchase #6 $237.26   12.152%   $266.0918   $266.09   $266.10   $266.10  
  Purchase #7 $38.99   3.548%   $40.3734   $40.37   $40.35   $40.40  
  7 purchase totals = $768.14       $830.0016   $829.99   $830.00   $829.90