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 $20.33   7.107%   $21.7749   $21.77   $21.75   $21.80  
  Purchase #2 $161.30   12.501%   $181.4641   $181.46   $181.45   $181.50  
  Purchase #3 $242.68   10.757%   $268.7851   $268.79   $268.80   $268.80  
  Purchase #4 $113.46   8.405%   $122.9963   $123.00   $123.00   $123.00  
  Purchase #5 $132.97   6.381%   $141.4548   $141.45   $141.45   $141.50  
  Purchase #6 $187.62   7.297%   $201.3106   $201.31   $201.30   $201.30  
  Purchase #7 $55.49   10.526%   $61.3309   $61.33   $61.35   $61.30  
  7 purchase totals = $913.85       $999.1167   $999.11   $999.10   $999.20