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 $11.85   8.739%   $12.8856   $12.89   $12.90   $12.90  
  Purchase #2 $173.35   4.851%   $181.7592   $181.76   $181.75   $181.80  
  Purchase #3 $73.90   7.711%   $79.5984   $79.60   $79.60   $79.60  
  Purchase #4 $129.73   12.596%   $146.0708   $146.07   $146.05   $146.10  
  Purchase #5 $135.58   8.136%   $146.6108   $146.61   $146.60   $146.60  
  Purchase #6 $68.63   7.700%   $73.9145   $73.91   $73.90   $73.90  
  Purchase #7 $171.44   4.415%   $179.0091   $179.01   $179.00   $179.00  
  7 purchase totals = $764.48       $819.8484   $819.85   $819.80   $819.90