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 $18.80   11.497%   $20.9614   $20.96   $20.95   $21.00  
  Purchase #2 $149.78   12.787%   $168.9324   $168.93   $168.95   $168.90  
  Purchase #3 $102.44   5.748%   $108.3283   $108.33   $108.35   $108.30  
  Purchase #4 $44.47   9.271%   $48.5928   $48.59   $48.60   $48.60  
  Purchase #5 $32.69   11.111%   $36.3222   $36.32   $36.30   $36.30  
  Purchase #6 $206.45   9.497%   $226.0566   $226.06   $226.05   $226.10  
  Purchase #7 $167.40   7.783%   $180.4287   $180.43   $180.45   $180.40  
  7 purchase totals = $722.03       $789.6224   $789.62   $789.65   $789.60