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 $27.31   6.461%   $29.0745   $29.07   $29.05   $29.10  
  Purchase #2 $39.96   12.589%   $44.9906   $44.99   $45.00   $45.00  
  Purchase #3 $220.93   9.696%   $242.3514   $242.35   $242.35   $242.40  
  Purchase #4 $128.27   12.516%   $144.3243   $144.32   $144.30   $144.30  
  Purchase #5 $171.93   13.335%   $194.8569   $194.86   $194.85   $194.90  
  Purchase #6 $79.77   9.692%   $87.5013   $87.50   $87.50   $87.50  
  Purchase #7 $235.42   11.422%   $262.3097   $262.31   $262.30   $262.30  
  7 purchase totals = $903.59       $1,005.4087   $1,005.40   $1,005.35   $1,005.50