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 $15.38   13.379%   $17.4377   $17.44   $17.45   $17.40  
  Purchase #2 $235.70   12.565%   $265.3157   $265.32   $265.30   $265.30  
  Purchase #3 $203.38   13.750%   $231.3448   $231.34   $231.35   $231.30  
  Purchase #4 $6.72   6.330%   $7.1454   $7.15   $7.15   $7.10  
  Purchase #5 $202.98   9.340%   $221.9383   $221.94   $221.95   $221.90  
  Purchase #6 $18.59   10.707%   $20.5804   $20.58   $20.60   $20.60  
  Purchase #7 $157.55   10.447%   $174.0092   $174.01   $174.00   $174.00  
  7 purchase totals = $840.30       $937.7715   $937.78   $937.80   $937.60