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 $18.65   9.424%   $20.4076   $20.41   $20.40   $20.40  
  Purchase #2 $165.74   3.421%   $171.41   $171.41   $171.40   $171.40  
  Purchase #3 $123.92   11.771%   $138.5066   $138.51   $138.50   $138.50  
  Purchase #4 $142.48   8.864%   $155.1094   $155.11   $155.10   $155.10  
  Purchase #5 $65.81   4.218%   $68.5859   $68.59   $68.60   $68.60  
  Purchase #6 $50.84   4.431%   $53.0927   $53.09   $53.10   $53.10  
  Purchase #7 $93.61   13.403%   $106.1565   $106.16   $106.15   $106.20  
  7 purchase totals = $661.05       $713.2687   $713.28   $713.25   $713.30