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 $133.40   4.128%   $138.9068   $138.91   $138.90   $138.90  
  Purchase #2 $127.29   8.835%   $138.5361   $138.54   $138.55   $138.50  
  Purchase #3 $14.89   6.840%   $15.9085   $15.91   $15.90   $15.90  
  Purchase #4 $60.78   9.169%   $66.3529   $66.35   $66.35   $66.40  
  Purchase #5 $143.77   8.513%   $156.0091   $156.01   $156.00   $156.00  
  Purchase #6 $84.27   5.532%   $88.9318   $88.93   $88.95   $88.90  
  Purchase #7 $90.38   8.161%   $97.7559   $97.76   $97.75   $97.80  
  7 purchase totals = $654.78       $702.4011   $702.41   $702.40   $702.40