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 $75.29   6.468%   $80.1598   $80.16   $80.15   $80.20  
  Purchase #2 $5.93   8.744%   $6.4485   $6.45   $6.45   $6.40  
  Purchase #3 $21.92   5.655%   $23.1596   $23.16   $23.15   $23.20  
  Purchase #4 $22.76   5.174%   $23.9376   $23.94   $23.95   $23.90  
  Purchase #5 $137.33   4.829%   $143.9617   $143.96   $143.95   $144.00  
  Purchase #6 $216.36   13.282%   $245.0969   $245.10   $245.10   $245.10  
  Purchase #7 $87.61   11.468%   $97.6571   $97.66   $97.65   $97.70  
  7 purchase totals = $567.20       $620.4212   $620.43   $620.40   $620.50