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 $47.57   5.600%   $50.2339   $50.23   $50.25   $50.20  
  Purchase #2 $80.53   11.102%   $89.4704   $89.47   $89.45   $89.50  
  Purchase #3 $7.31   11.598%   $8.1578   $8.16   $8.15   $8.20  
  Purchase #4 $174.94   8.390%   $189.6175   $189.62   $189.60   $189.60  
  Purchase #5 $112.98   9.777%   $124.0261   $124.03   $124.05   $124.00  
  Purchase #6 $76.99   12.781%   $86.8301   $86.83   $86.85   $86.80  
  Purchase #7 $9.47   6.791%   $10.1131   $10.11   $10.10   $10.10  
  7 purchase totals = $509.79       $558.4489   $558.45   $558.45   $558.40