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 $244.54   11.773%   $273.3297   $273.33   $273.35   $273.30  
  Purchase #2 $109.60   6.437%   $116.655   $116.66   $116.65   $116.70  
  Purchase #3 $128.39   10.831%   $142.2959   $142.30   $142.30   $142.30  
  Purchase #4 $155.79   3.695%   $161.5464   $161.55   $161.55   $161.50  
  Purchase #5 $126.99   7.335%   $136.3047   $136.30   $136.30   $136.30  
  Purchase #6 $209.49   3.833%   $217.5198   $217.52   $217.50   $217.50  
  Purchase #7 $9.73   13.667%   $11.0598   $11.06   $11.05   $11.10  
  7 purchase totals = $984.53       $1,058.7113   $1,058.72   $1,058.70   $1,058.70