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 $19.75   6.132%   $20.9611   $20.96   $20.95   $21.00  
  Purchase #2 $93.74   6.474%   $99.8087   $99.81   $99.80   $99.80  
  Purchase #3 $115.39   6.512%   $122.9042   $122.90   $122.90   $122.90  
  Purchase #4 $227.66   11.700%   $254.2962   $254.30   $254.30   $254.30  
  Purchase #5 $143.49   13.518%   $162.887   $162.89   $162.90   $162.90  
  Purchase #6 $68.86   7.510%   $74.0314   $74.03   $74.05   $74.00  
  Purchase #7 $122.48   8.485%   $132.8724   $132.87   $132.85   $132.90  
  7 purchase totals = $791.37       $867.7610   $867.76   $867.75   $867.80