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 $132.38   4.500%   $138.3371   $138.34   $138.35   $138.30  
  Purchase #2 $137.59   12.836%   $155.2511   $155.25   $155.25   $155.30  
  Purchase #3 $13.94   8.396%   $15.1104   $15.11   $15.10   $15.10  
  Purchase #4 $109.26   10.562%   $120.8   $120.80   $120.80   $120.80  
  Purchase #5 $13.31   6.546%   $14.1813   $14.18   $14.20   $14.20  
  Purchase #6 $192.66   10.492%   $212.8739   $212.87   $212.85   $212.90  
  Purchase #7 $22.28   4.706%   $23.3285   $23.33   $23.35   $23.30  
  7 purchase totals = $621.42       $679.8823   $679.88   $679.90   $679.90