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 $17.31   7.818%   $18.6633   $18.66   $18.65   $18.70  
  Purchase #2 $134.52   4.668%   $140.7994   $140.80   $140.80   $140.80  
  Purchase #3 $50.33   13.271%   $57.0093   $57.01   $57.00   $57.00  
  Purchase #4 $105.38   3.696%   $109.2748   $109.27   $109.25   $109.30  
  Purchase #5 $133.97   3.739%   $138.9791   $138.98   $139.00   $139.00  
  Purchase #6 $162.41   10.849%   $180.0299   $180.03   $180.05   $180.00  
  Purchase #7 $161.67   6.313%   $171.8762   $171.88   $171.90   $171.90  
  7 purchase totals = $765.59       $816.6320   $816.63   $816.65   $816.70