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 $37.31   6.468%   $39.7232   $39.72   $39.70   $39.70  
  Purchase #2 $63.41   10.826%   $70.2748   $70.27   $70.25   $70.30  
  Purchase #3 $18.54   5.355%   $19.5328   $19.53   $19.55   $19.50  
  Purchase #4 $7.58   5.759%   $8.0165   $8.02   $8.00   $8.00  
  Purchase #5 $90.29   9.505%   $98.8721   $98.87   $98.85   $98.90  
  Purchase #6 $86.68   7.373%   $93.0709   $93.07   $93.05   $93.10  
  Purchase #7 $80.38   13.172%   $90.9677   $90.97   $90.95   $91.00  
  7 purchase totals = $384.19       $420.4580   $420.45   $420.35   $420.50