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 $20.37   5.444%   $21.4789   $21.48   $21.50   $21.50  
  Purchase #2 $27.33   7.154%   $29.2852   $29.29   $29.30   $29.30  
  Purchase #3 $91.50   13.430%   $103.7885   $103.79   $103.80   $103.80  
  Purchase #4 $209.99   9.573%   $230.0923   $230.09   $230.10   $230.10  
  Purchase #5 $122.99   12.812%   $138.7475   $138.75   $138.75   $138.70  
  Purchase #6 $112.71   7.565%   $121.2365   $121.24   $121.25   $121.20  
  Purchase #7 $236.41   12.262%   $265.3986   $265.40   $265.40   $265.40  
  7 purchase totals = $821.30       $910.0275   $910.04   $910.10   $910.00