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 $162.79   10.819%   $180.4023   $180.40   $180.40   $180.40  
  Purchase #2 $124.96   10.867%   $138.5394   $138.54   $138.55   $138.50  
  Purchase #3 $85.92   6.182%   $91.2316   $91.23   $91.25   $91.20  
  Purchase #4 $7.39   8.152%   $7.9924   $7.99   $8.00   $8.00  
  Purchase #5 $89.92   10.340%   $99.2177   $99.22   $99.20   $99.20  
  Purchase #6 $73.68   11.501%   $82.1539   $82.15   $82.15   $82.20  
  Purchase #7 $40.86   9.712%   $44.8283   $44.83   $44.85   $44.80  
  7 purchase totals = $585.52       $644.3656   $644.36   $644.40   $644.30