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 $19.83   6.192%   $21.0579   $21.06   $21.05   $21.10  
  Purchase #2 $12.29   4.595%   $12.8547   $12.85   $12.85   $12.90  
  Purchase #3 $199.67   11.153%   $221.9392   $221.94   $221.95   $221.90  
  Purchase #4 $29.36   7.120%   $31.4504   $31.45   $31.45   $31.50  
  Purchase #5 $103.70   5.651%   $109.5601   $109.56   $109.55   $109.60  
  Purchase #6 $31.52   6.484%   $33.5638   $33.56   $33.55   $33.60  
  Purchase #7 $183.63   10.375%   $202.6816   $202.68   $202.70   $202.70  
  7 purchase totals = $580.00       $633.1077   $633.10   $633.10   $633.30