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 $6.72   3.852%   $6.9789   $6.98   $7.00   $7.00  
  Purchase #2 $5.66   9.582%   $6.2023   $6.20   $6.20   $6.20  
  Purchase #3 $240.73   6.409%   $256.1584   $256.16   $256.15   $256.20  
  Purchase #4 $185.66   11.713%   $207.4064   $207.41   $207.40   $207.40  
  Purchase #5 $32.75   11.596%   $36.5477   $36.55   $36.55   $36.50  
  Purchase #6 $104.92   10.724%   $116.1716   $116.17   $116.15   $116.20  
  Purchase #7 $247.28   9.864%   $271.6717   $271.67   $271.65   $271.70  
  7 purchase totals = $823.72       $901.1370   $901.14   $901.10   $901.20