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 $19.96   4.288%   $20.8159   $20.82   $20.80   $20.80  
  Purchase #2 $149.49   10.717%   $165.5108   $165.51   $165.50   $165.50  
  Purchase #3 $94.82   8.771%   $103.1367   $103.14   $103.15   $103.10  
  Purchase #4 $121.45   3.368%   $125.5404   $125.54   $125.55   $125.50  
  Purchase #5 $197.60   7.649%   $212.7144   $212.71   $212.70   $212.70  
  Purchase #6 $73.69   7.737%   $79.3914   $79.39   $79.40   $79.40  
  Purchase #7 $107.90   6.463%   $114.8736   $114.87   $114.85   $114.90  
  7 purchase totals = $764.91       $821.9832   $821.98   $821.95   $821.90