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 $36.62   7.551%   $39.3852   $39.39   $39.40   $39.40  
  Purchase #2 $28.44   8.200%   $30.7721   $30.77   $30.75   $30.80  
  Purchase #3 $192.71   4.590%   $201.5554   $201.56   $201.55   $201.60  
  Purchase #4 $230.53   8.107%   $249.2191   $249.22   $249.20   $249.20  
  Purchase #5 $30.53   5.324%   $32.1554   $32.16   $32.15   $32.20  
  Purchase #6 $61.59   7.345%   $66.1138   $66.11   $66.10   $66.10  
  Purchase #7 $61.64   13.231%   $69.7956   $69.80   $69.80   $69.80  
  7 purchase totals = $642.06       $688.9966   $689.01   $688.95   $689.10