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 $9.85   8.178%   $10.6555   $10.66   $10.65   $10.70  
  Purchase #2 $5.52   3.642%   $5.721   $5.72   $5.70   $5.70  
  Purchase #3 $154.51   13.153%   $174.8327   $174.83   $174.85   $174.80  
  Purchase #4 $30.35   11.655%   $33.8873   $33.89   $33.90   $33.90  
  Purchase #5 $125.61   12.242%   $140.9872   $140.99   $141.00   $141.00  
  Purchase #6 $47.78   13.274%   $54.1223   $54.12   $54.10   $54.10  
  Purchase #7 $207.82   6.120%   $220.5386   $220.54   $220.55   $220.50  
  7 purchase totals = $581.44       $640.7446   $640.75   $640.75   $640.70