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 $7.40   5.758%   $7.8261   $7.83   $7.85   $7.80  
  Purchase #2 $84.56   3.586%   $87.5923   $87.59   $87.60   $87.60  
  Purchase #3 $146.33   8.590%   $158.8997   $158.90   $158.90   $158.90  
  Purchase #4 $57.43   4.160%   $59.8191   $59.82   $59.80   $59.80  
  Purchase #5 $177.39   13.675%   $201.6481   $201.65   $201.65   $201.60  
  Purchase #6 $228.99   5.723%   $242.0951   $242.10   $242.10   $242.10  
  Purchase #7 $75.35   9.391%   $82.4261   $82.43   $82.45   $82.40  
  7 purchase totals = $777.45       $840.3065   $840.32   $840.35   $840.20