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 $91.37   8.253%   $98.9108   $98.91   $98.90   $98.90  
  Purchase #2 $122.85   7.330%   $131.8549   $131.85   $131.85   $131.90  
  Purchase #3 $142.42   13.417%   $161.5285   $161.53   $161.55   $161.50  
  Purchase #4 $107.66   10.353%   $118.806   $118.81   $118.80   $118.80  
  Purchase #5 $153.91   11.731%   $171.9652   $171.97   $171.95   $172.00  
  Purchase #6 $60.88   7.231%   $65.2822   $65.28   $65.30   $65.30  
  Purchase #7 $115.66   11.731%   $129.2281   $129.23   $129.25   $129.20  
  7 purchase totals = $794.75       $877.5757   $877.58   $877.60   $877.60