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 $135.68   11.380%   $151.1204   $151.12   $151.10   $151.10  
  Purchase #2 $35.97   11.513%   $40.1112   $40.11   $40.10   $40.10  
  Purchase #3 $242.52   3.184%   $250.2418   $250.24   $250.25   $250.20  
  Purchase #4 $167.84   12.183%   $188.2879   $188.29   $188.30   $188.30  
  Purchase #5 $112.76   13.211%   $127.6567   $127.66   $127.65   $127.70  
  Purchase #6 $159.77   13.229%   $180.906   $180.91   $180.90   $180.90  
  Purchase #7 $181.99   5.782%   $192.5127   $192.51   $192.50   $192.50  
  7 purchase totals = $1,036.53       $1,130.8367   $1,130.84   $1,130.80   $1,130.80