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 $19.89   4.262%   $20.7377   $20.74   $20.75   $20.70  
  Purchase #2 $22.69   3.617%   $23.5107   $23.51   $23.50   $23.50  
  Purchase #3 $123.71   11.851%   $138.3709   $138.37   $138.35   $138.40  
  Purchase #4 $39.58   9.598%   $43.3789   $43.38   $43.40   $43.40  
  Purchase #5 $22.36   3.868%   $23.2249   $23.22   $23.20   $23.20  
  Purchase #6 $65.36   3.400%   $67.5822   $67.58   $67.60   $67.60  
  Purchase #7 $36.94   4.502%   $38.603   $38.60   $38.60   $38.60  
  7 purchase totals = $330.53       $355.4083   $355.40   $355.40   $355.40