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 $19.98   4.499%   $20.8789   $20.88   $20.90   $20.90  
  Purchase #2 $124.30   9.583%   $136.2117   $136.21   $136.20   $136.20  
  Purchase #3 $160.75   3.838%   $166.9196   $166.92   $166.90   $166.90  
  Purchase #4 $97.26   5.678%   $102.7824   $102.78   $102.80   $102.80  
  Purchase #5 $47.66   9.639%   $52.2539   $52.25   $52.25   $52.30  
  Purchase #6 $93.74   6.197%   $99.5491   $99.55   $99.55   $99.50  
  Purchase #7 $87.28   8.402%   $94.6133   $94.61   $94.60   $94.60  
  7 purchase totals = $630.97       $673.2089   $673.20   $673.20   $673.20