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 $9.93   6.192%   $10.5449   $10.54   $10.55   $10.50  
  Purchase #2 $13.54   13.737%   $15.4   $15.40   $15.40   $15.40  
  Purchase #3 $245.34   3.266%   $253.3528   $253.35   $253.35   $253.40  
  Purchase #4 $116.74   9.623%   $127.9739   $127.97   $127.95   $128.00  
  Purchase #5 $194.64   4.275%   $202.9609   $202.96   $202.95   $203.00  
  Purchase #6 $12.59   5.259%   $13.2521   $13.25   $13.25   $13.30  
  Purchase #7 $13.77   11.424%   $15.3431   $15.34   $15.35   $15.30  
  7 purchase totals = $606.55       $638.8277   $638.81   $638.80   $638.90