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 $19.97   6.536%   $21.2752   $21.28   $21.30   $21.30  
  Purchase #2 $24.96   12.320%   $28.0351   $28.04   $28.05   $28.00  
  Purchase #3 $124.74   9.264%   $136.2959   $136.30   $136.30   $136.30  
  Purchase #4 $91.45   11.551%   $102.0134   $102.01   $102.00   $102.00  
  Purchase #5 $26.74   4.309%   $27.8922   $27.89   $27.90   $27.90  
  Purchase #6 $95.65   13.400%   $108.4671   $108.47   $108.45   $108.50  
  Purchase #7 $141.59   4.527%   $147.9998   $148.00   $148.00   $148.00  
  7 purchase totals = $525.10       $571.9787   $571.99   $572.00   $572.00