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 $27.35   9.125%   $29.8457   $29.85   $29.85   $29.80  
  Purchase #2 $126.78   6.317%   $134.7887   $134.79   $134.80   $134.80  
  Purchase #3 $149.56   13.744%   $170.1155   $170.12   $170.10   $170.10  
  Purchase #4 $108.36   12.398%   $121.7945   $121.79   $121.80   $121.80  
  Purchase #5 $195.57   11.748%   $218.5456   $218.55   $218.55   $218.50  
  Purchase #6 $106.85   4.103%   $111.2341   $111.23   $111.25   $111.20  
  Purchase #7 $22.34   3.493%   $23.1203   $23.12   $23.10   $23.10  
  7 purchase totals = $736.81       $809.4444   $809.45   $809.45   $809.30