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 $241.93   12.227%   $271.5108   $271.51   $271.50   $271.50  
  Purchase #2 $212.86   8.197%   $230.3081   $230.31   $230.30   $230.30  
  Purchase #3 $8.66   9.832%   $9.5115   $9.51   $9.50   $9.50  
  Purchase #4 $48.35   7.526%   $51.9888   $51.99   $52.00   $52.00  
  Purchase #5 $175.39   6.260%   $186.3694   $186.37   $186.35   $186.40  
  Purchase #6 $93.30   13.526%   $105.9198   $105.92   $105.90   $105.90  
  Purchase #7 $118.61   7.812%   $127.8758   $127.88   $127.90   $127.90  
  7 purchase totals = $899.10       $983.4842   $983.49   $983.45   $983.50