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 $242.66   12.653%   $273.3638   $273.36   $273.35   $273.40  
  Purchase #2 $218.67   11.256%   $243.2835   $243.28   $243.30   $243.30  
  Purchase #3 $178.45   8.863%   $194.266   $194.27   $194.25   $194.30  
  Purchase #4 $126.74   4.358%   $132.2633   $132.26   $132.25   $132.30  
  Purchase #5 $174.25   9.695%   $191.1435   $191.14   $191.15   $191.10  
  Purchase #6 $177.51   3.106%   $183.0235   $183.02   $183.00   $183.00  
  Purchase #7 $36.85   10.613%   $40.7609   $40.76   $40.75   $40.80  
  7 purchase totals = $1,155.13       $1,258.1045   $1,258.09   $1,258.05   $1,258.20