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 $18.45   7.722%   $19.8747   $19.87   $19.85   $19.90  
  Purchase #2 $25.97   9.601%   $28.4634   $28.46   $28.45   $28.50  
  Purchase #3 $148.94   8.860%   $162.1361   $162.14   $162.15   $162.10  
  Purchase #4 $131.57   9.606%   $144.2086   $144.21   $144.20   $144.20  
  Purchase #5 $156.56   11.278%   $174.2168   $174.22   $174.20   $174.20  
  Purchase #6 $181.79   7.492%   $195.4097   $195.41   $195.40   $195.40  
  Purchase #7 $81.81   4.463%   $85.4612   $85.46   $85.45   $85.50  
  7 purchase totals = $745.09       $809.7705   $809.77   $809.70   $809.80