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 $29.80   5.744%   $31.5117   $31.51   $31.50   $31.50  
  Purchase #2 $154.78   3.681%   $160.4775   $160.48   $160.50   $160.50  
  Purchase #3 $27.36   12.335%   $30.7349   $30.73   $30.75   $30.70  
  Purchase #4 $43.96   3.561%   $45.5254   $45.53   $45.55   $45.50  
  Purchase #5 $199.60   9.309%   $218.1808   $218.18   $218.20   $218.20  
  Purchase #6 $154.64   10.293%   $170.5571   $170.56   $170.55   $170.60  
  Purchase #7 $228.44   11.723%   $255.22   $255.22   $255.20   $255.20  
  7 purchase totals = $838.58       $912.2074   $912.21   $912.25   $912.20