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 $45.38   9.222%   $49.5649   $49.56   $49.55   $49.60  
  Purchase #2 $182.38   12.752%   $205.6371   $205.64   $205.65   $205.60  
  Purchase #3 $93.77   3.800%   $97.3333   $97.33   $97.35   $97.30  
  Purchase #4 $211.80   10.329%   $233.6768   $233.68   $233.70   $233.70  
  Purchase #5 $156.88   12.798%   $176.9575   $176.96   $176.95   $177.00  
  Purchase #6 $186.82   3.744%   $193.8145   $193.81   $193.80   $193.80  
  Purchase #7 $12.98   11.607%   $14.4866   $14.49   $14.50   $14.50  
  7 purchase totals = $890.01       $971.4707   $971.47   $971.50   $971.50