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 $180.47   13.463%   $204.7667   $204.77   $204.75   $204.80  
  Purchase #2 $26.92   11.750%   $30.0831   $30.08   $30.10   $30.10  
  Purchase #3 $244.90   12.518%   $275.5566   $275.56   $275.55   $275.60  
  Purchase #4 $198.49   12.299%   $222.9023   $222.90   $222.90   $222.90  
  Purchase #5 $115.89   6.146%   $123.0126   $123.01   $123.00   $123.00  
  Purchase #6 $31.53   4.620%   $32.9867   $32.99   $33.00   $33.00  
  Purchase #7 $103.51   6.704%   $110.4493   $110.45   $110.45   $110.40  
  7 purchase totals = $901.71       $999.7573   $999.76   $999.75   $999.80