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 $105.41   3.177%   $108.7589   $108.76   $108.75   $108.80  
  Purchase #2 $151.55   6.389%   $161.2325   $161.23   $161.25   $161.20  
  Purchase #3 $93.87   9.830%   $103.0974   $103.10   $103.10   $103.10  
  Purchase #4 $35.51   10.778%   $39.3373   $39.34   $39.35   $39.30  
  Purchase #5 $33.54   12.570%   $37.756   $37.76   $37.75   $37.80  
  Purchase #6 $121.80   12.533%   $137.0652   $137.07   $137.05   $137.10  
  Purchase #7 $47.54   6.726%   $50.7375   $50.74   $50.75   $50.70  
  7 purchase totals = $589.22       $637.9848   $638.00   $638.00   $638.00