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 $15.72   13.849%   $17.8971   $17.90   $17.90   $17.90  
  Purchase #2 $8.70   8.371%   $9.4283   $9.43   $9.45   $9.40  
  Purchase #3 $228.82   10.426%   $252.6768   $252.68   $252.70   $252.70  
  Purchase #4 $136.29   4.127%   $141.9147   $141.91   $141.90   $141.90  
  Purchase #5 $203.96   6.531%   $217.2806   $217.28   $217.30   $217.30  
  Purchase #6 $43.99   7.633%   $47.3478   $47.35   $47.35   $47.30  
  Purchase #7 $115.65   6.865%   $123.5894   $123.59   $123.60   $123.60  
  7 purchase totals = $753.13       $810.1347   $810.14   $810.20   $810.10