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 $20.99   5.330%   $22.1088   $22.11   $22.10   $22.10  
  Purchase #2 $9.58   4.455%   $10.0068   $10.01   $10.00   $10.00  
  Purchase #3 $229.30   7.126%   $245.6399   $245.64   $245.65   $245.60  
  Purchase #4 $61.57   8.801%   $66.9888   $66.99   $67.00   $67.00  
  Purchase #5 $94.32   6.204%   $100.1716   $100.17   $100.15   $100.20  
  Purchase #6 $162.40   13.352%   $184.0836   $184.08   $184.10   $184.10  
  Purchase #7 $172.48   10.127%   $189.947   $189.95   $189.95   $189.90  
  7 purchase totals = $750.64       $818.9465   $818.95   $818.95   $818.90