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 $246.58   11.827%   $275.743   $275.74   $275.75   $275.70  
  Purchase #2 $67.74   10.106%   $74.5858   $74.59   $74.60   $74.60  
  Purchase #3 $159.28   5.574%   $168.1583   $168.16   $168.15   $168.20  
  Purchase #4 $129.78   6.434%   $138.13   $138.13   $138.15   $138.10  
  Purchase #5 $126.41   11.755%   $141.2695   $141.27   $141.25   $141.30  
  Purchase #6 $22.42   13.452%   $25.4359   $25.44   $25.45   $25.40  
  Purchase #7 $32.72   3.468%   $33.8547   $33.85   $33.85   $33.90  
  7 purchase totals = $784.93       $857.1772   $857.18   $857.20   $857.20