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 $110.68   12.253%   $124.2416   $124.24   $124.25   $124.20  
  Purchase #2 $97.79   8.111%   $105.7217   $105.72   $105.70   $105.70  
  Purchase #3 $169.76   7.567%   $182.6057   $182.61   $182.60   $182.60  
  Purchase #4 $77.37   3.326%   $79.9433   $79.94   $79.95   $79.90  
  Purchase #5 $153.31   4.730%   $160.5616   $160.56   $160.55   $160.60  
  Purchase #6 $97.82   3.204%   $100.9542   $100.95   $100.95   $101.00  
  Purchase #7 $14.39   3.840%   $14.9426   $14.94   $14.95   $14.90  
  7 purchase totals = $721.12       $768.9707   $768.96   $768.95   $768.90