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 $19.87   3.506%   $20.5666   $20.57   $20.55   $20.60  
  Purchase #2 $138.44   5.190%   $145.625   $145.63   $145.65   $145.60  
  Purchase #3 $39.92   10.404%   $44.0733   $44.07   $44.05   $44.10  
  Purchase #4 $183.60   10.426%   $202.7421   $202.74   $202.75   $202.70  
  Purchase #5 $214.85   11.384%   $239.3085   $239.31   $239.30   $239.30  
  Purchase #6 $139.92   8.717%   $152.1168   $152.12   $152.10   $152.10  
  Purchase #7 $75.64   12.337%   $84.9717   $84.97   $84.95   $85.00  
  7 purchase totals = $812.24       $889.4040   $889.41   $889.35   $889.40