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 $85.82   6.295%   $91.2224   $91.22   $91.20   $91.20  
  Purchase #2 $179.77   3.258%   $185.6269   $185.63   $185.65   $185.60  
  Purchase #3 $8.58   8.754%   $9.3311   $9.33   $9.35   $9.30  
  Purchase #4 $133.39   3.698%   $138.3228   $138.32   $138.30   $138.30  
  Purchase #5 $55.30   8.680%   $60.1   $60.10   $60.10   $60.10  
  Purchase #6 $220.80   9.837%   $242.5201   $242.52   $242.50   $242.50  
  Purchase #7 $136.50   3.104%   $140.737   $140.74   $140.75   $140.70  
  7 purchase totals = $820.16       $867.8603   $867.86   $867.85   $867.70