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 $20.39   3.755%   $21.1556   $21.16   $21.15   $21.20  
  Purchase #2 $150.34   8.326%   $162.8573   $162.86   $162.85   $162.90  
  Purchase #3 $170.89   4.842%   $179.1645   $179.16   $179.15   $179.20  
  Purchase #4 $146.73   10.386%   $161.9694   $161.97   $161.95   $162.00  
  Purchase #5 $233.48   7.544%   $251.0937   $251.09   $251.10   $251.10  
  Purchase #6 $161.57   8.117%   $174.6846   $174.68   $174.70   $174.70  
  Purchase #7 $168.99   9.462%   $184.9798   $184.98   $185.00   $185.00  
  7 purchase totals = $1,052.39       $1,135.9049   $1,135.90   $1,135.90   $1,136.10