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 $17.60   3.832%   $18.2744   $18.27   $18.25   $18.30  
  Purchase #2 $7.80   12.740%   $8.7937   $8.79   $8.80   $8.80  
  Purchase #3 $84.48   6.481%   $89.9551   $89.96   $89.95   $90.00  
  Purchase #4 $106.62   13.418%   $120.9263   $120.93   $120.95   $120.90  
  Purchase #5 $109.56   11.427%   $122.0794   $122.08   $122.10   $122.10  
  Purchase #6 $86.34   6.665%   $92.0946   $92.09   $92.10   $92.10  
  Purchase #7 $18.72   10.809%   $20.7434   $20.74   $20.75   $20.70  
  7 purchase totals = $431.12       $472.8669   $472.86   $472.90   $472.90