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 $18.90   5.724%   $19.9818   $19.98   $20.00   $20.00  
  Purchase #2 $79.68   12.729%   $89.8225   $89.82   $89.80   $89.80  
  Purchase #3 $197.61   3.787%   $205.0935   $205.09   $205.10   $205.10  
  Purchase #4 $112.37   3.620%   $116.4378   $116.44   $116.45   $116.40  
  Purchase #5 $201.54   12.160%   $226.0473   $226.05   $226.05   $226.00  
  Purchase #6 $223.55   7.801%   $240.9891   $240.99   $241.00   $241.00  
  Purchase #7 $145.28   10.384%   $160.3659   $160.37   $160.35   $160.40  
  7 purchase totals = $978.93       $1,058.7379   $1,058.74   $1,058.75   $1,058.70