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 $242.53   12.828%   $273.6417   $273.64   $273.65   $273.60  
  Purchase #2 $92.99   10.683%   $102.9241   $102.92   $102.90   $102.90  
  Purchase #3 $160.55   7.506%   $172.6009   $172.60   $172.60   $172.60  
  Purchase #4 $98.37   10.184%   $108.388   $108.39   $108.40   $108.40  
  Purchase #5 $54.93   7.327%   $58.9547   $58.95   $58.95   $59.00  
  Purchase #6 $206.61   5.565%   $218.1078   $218.11   $218.10   $218.10  
  Purchase #7 $88.39   11.146%   $98.2419   $98.24   $98.25   $98.20  
  7 purchase totals = $944.37       $1,032.8591   $1,032.85   $1,032.85   $1,032.80