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 $16.35   5.344%   $17.2237   $17.22   $17.20   $17.20  
  Purchase #2 $17.93   8.804%   $19.5086   $19.51   $19.50   $19.50  
  Purchase #3 $6.89   3.242%   $7.1134   $7.11   $7.10   $7.10  
  Purchase #4 $78.40   9.145%   $85.5697   $85.57   $85.55   $85.60  
  Purchase #5 $128.51   9.368%   $140.5488   $140.55   $140.55   $140.50  
  Purchase #6 $212.31   7.645%   $228.5411   $228.54   $228.55   $228.50  
  Purchase #7 $144.58   4.592%   $151.2191   $151.22   $151.20   $151.20  
  7 purchase totals = $604.97       $649.7244   $649.72   $649.65   $649.60