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 $18.84   7.221%   $20.2004   $20.20   $20.20   $20.20  
  Purchase #2 $19.70   4.215%   $20.5304   $20.53   $20.55   $20.50  
  Purchase #3 $140.99   10.303%   $155.5162   $155.52   $155.50   $155.50  
  Purchase #4 $177.97   4.106%   $185.2774   $185.28   $185.30   $185.30  
  Purchase #5 $101.85   9.430%   $111.4545   $111.45   $111.45   $111.50  
  Purchase #6 $75.57   4.668%   $79.0976   $79.10   $79.10   $79.10  
  Purchase #7 $87.28   11.844%   $97.6174   $97.62   $97.60   $97.60  
  7 purchase totals = $622.20       $669.6939   $669.70   $669.70   $669.70