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 $157.62   13.154%   $178.3533   $178.35   $178.35   $178.40  
  Purchase #2 $118.69   13.367%   $134.5553   $134.56   $134.55   $134.60  
  Purchase #3 $181.68   8.697%   $197.4807   $197.48   $197.50   $197.50  
  Purchase #4 $62.63   7.243%   $67.1663   $67.17   $67.15   $67.20  
  Purchase #5 $238.43   5.526%   $251.6056   $251.61   $251.60   $251.60  
  Purchase #6 $112.46   8.368%   $121.8707   $121.87   $121.85   $121.90  
  Purchase #7 $14.57   13.707%   $16.5671   $16.57   $16.55   $16.60  
  7 purchase totals = $886.08       $967.5990   $967.61   $967.55   $967.80