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 $5.44   7.431%   $5.8442   $5.84   $5.85   $5.80  
  Purchase #2 $25.71   12.173%   $28.8397   $28.84   $28.85   $28.80  
  Purchase #3 $136.29   3.178%   $140.6213   $140.62   $140.60   $140.60  
  Purchase #4 $133.44   10.233%   $147.0949   $147.09   $147.10   $147.10  
  Purchase #5 $190.66   3.861%   $198.0214   $198.02   $198.00   $198.00  
  Purchase #6 $47.99   3.336%   $49.5909   $49.59   $49.60   $49.60  
  Purchase #7 $63.80   9.113%   $69.6141   $69.61   $69.60   $69.60  
  7 purchase totals = $603.33       $639.6265   $639.61   $639.60   $639.50