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 $19.69   7.327%   $21.1327   $21.13   $21.15   $21.10  
  Purchase #2 $109.60   9.848%   $120.3934   $120.39   $120.40   $120.40  
  Purchase #3 $202.63   3.830%   $210.3907   $210.39   $210.40   $210.40  
  Purchase #4 $72.51   9.772%   $79.5957   $79.60   $79.60   $79.60  
  Purchase #5 $241.89   8.822%   $263.2295   $263.23   $263.25   $263.20  
  Purchase #6 $164.79   9.463%   $180.3841   $180.38   $180.40   $180.40  
  Purchase #7 $104.49   6.424%   $111.2024   $111.20   $111.20   $111.20  
  7 purchase totals = $915.60       $986.3285   $986.32   $986.40   $986.30