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 $12.52   11.172%   $13.9187   $13.92   $13.90   $13.90  
  Purchase #2 $10.80   11.523%   $12.0445   $12.04   $12.05   $12.00  
  Purchase #3 $5.80   9.312%   $6.3401   $6.34   $6.35   $6.30  
  Purchase #4 $173.82   13.326%   $196.9833   $196.98   $197.00   $197.00  
  Purchase #5 $101.78   8.811%   $110.7478   $110.75   $110.75   $110.70  
  Purchase #6 $75.61   6.105%   $80.226   $80.23   $80.25   $80.20  
  Purchase #7 $6.33   7.553%   $6.8081   $6.81   $6.80   $6.80  
  7 purchase totals = $386.66       $427.0685   $427.07   $427.10   $426.90