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 $167.97   6.197%   $178.3791   $178.38   $178.40   $178.40  
  Purchase #2 $147.76   10.105%   $162.6911   $162.69   $162.70   $162.70  
  Purchase #3 $178.50   13.301%   $202.2423   $202.24   $202.25   $202.20  
  Purchase #4 $36.51   8.473%   $39.6035   $39.60   $39.60   $39.60  
  Purchase #5 $35.50   7.736%   $38.2463   $38.25   $38.25   $38.20  
  Purchase #6 $12.46   4.364%   $13.0038   $13.00   $13.00   $13.00  
  Purchase #7 $13.87   6.664%   $14.7943   $14.79   $14.80   $14.80  
  7 purchase totals = $592.57       $648.9604   $648.95   $649.00   $648.90