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 $242.46   13.423%   $275.0054   $275.01   $275.00   $275.00  
  Purchase #2 $54.76   12.721%   $61.726   $61.73   $61.75   $61.70  
  Purchase #3 $151.48   12.597%   $170.5619   $170.56   $170.55   $170.60  
  Purchase #4 $235.30   7.301%   $252.4793   $252.48   $252.50   $252.50  
  Purchase #5 $203.77   4.195%   $212.3182   $212.32   $212.30   $212.30  
  Purchase #6 $127.70   3.174%   $131.7532   $131.75   $131.75   $131.80  
  Purchase #7 $232.35   9.500%   $254.4233   $254.42   $254.40   $254.40  
  7 purchase totals = $1,247.82       $1,358.2673   $1,358.27   $1,358.25   $1,358.30