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 $118.96   13.204%   $134.6675   $134.67   $134.65   $134.70  
  Purchase #2 $6.52   10.432%   $7.2002   $7.20   $7.20   $7.20  
  Purchase #3 $26.33   4.408%   $27.4906   $27.49   $27.50   $27.50  
  Purchase #4 $131.44   9.233%   $143.5759   $143.58   $143.60   $143.60  
  Purchase #5 $36.50   8.576%   $39.6302   $39.63   $39.65   $39.60  
  Purchase #6 $208.60   3.273%   $215.4275   $215.43   $215.45   $215.40  
  Purchase #7 $104.45   7.588%   $112.3757   $112.38   $112.40   $112.40  
  7 purchase totals = $632.80       $680.3676   $680.38   $680.45   $680.40