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 $19.42   7.199%   $20.818   $20.82   $20.80   $20.80  
  Purchase #2 $47.90   3.782%   $49.7116   $49.71   $49.70   $49.70  
  Purchase #3 $114.90   6.270%   $122.1042   $122.10   $122.10   $122.10  
  Purchase #4 $6.49   7.354%   $6.9673   $6.97   $6.95   $7.00  
  Purchase #5 $94.83   10.424%   $104.7151   $104.72   $104.70   $104.70  
  Purchase #6 $179.85   4.301%   $187.5853   $187.59   $187.60   $187.60  
  Purchase #7 $42.96   5.608%   $45.3692   $45.37   $45.35   $45.40  
  7 purchase totals = $506.35       $537.2707   $537.28   $537.20   $537.30