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 $44.98   10.777%   $49.8275   $49.83   $49.85   $49.80  
  Purchase #2 $146.92   4.291%   $153.2243   $153.22   $153.20   $153.20  
  Purchase #3 $172.32   4.753%   $180.5104   $180.51   $180.50   $180.50  
  Purchase #4 $138.41   13.726%   $157.4082   $157.41   $157.40   $157.40  
  Purchase #5 $74.57   11.257%   $82.9643   $82.96   $82.95   $83.00  
  Purchase #6 $91.69   12.662%   $103.2998   $103.30   $103.30   $103.30  
  Purchase #7 $250.82   10.519%   $277.2038   $277.20   $277.20   $277.20  
  7 purchase totals = $919.71       $1,004.4383   $1,004.43   $1,004.40   $1,004.40