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 $220.53   9.852%   $242.2566   $242.26   $242.25   $242.30  
  Purchase #2 $131.75   6.532%   $140.3559   $140.36   $140.35   $140.40  
  Purchase #3 $141.37   5.684%   $149.4055   $149.41   $149.40   $149.40  
  Purchase #4 $111.82   5.720%   $118.2161   $118.22   $118.20   $118.20  
  Purchase #5 $42.26   6.852%   $45.1557   $45.16   $45.15   $45.20  
  Purchase #6 $24.93   3.527%   $25.8093   $25.81   $25.80   $25.80  
  Purchase #7 $89.65   9.389%   $98.0672   $98.07   $98.05   $98.10  
  7 purchase totals = $762.31       $819.2663   $819.29   $819.20   $819.40