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 $24.33   3.200%   $25.1086   $25.11   $25.10   $25.10  
  Purchase #2 $14.81   6.602%   $15.7878   $15.79   $15.80   $15.80  
  Purchase #3 $12.77   8.330%   $13.8337   $13.83   $13.85   $13.80  
  Purchase #4 $230.45   12.218%   $258.6064   $258.61   $258.60   $258.60  
  Purchase #5 $23.53   11.331%   $26.1962   $26.20   $26.20   $26.20  
  Purchase #6 $135.52   11.744%   $151.4355   $151.44   $151.45   $151.40  
  Purchase #7 $191.51   4.362%   $199.8637   $199.86   $199.85   $199.90  
  7 purchase totals = $632.92       $690.8319   $690.84   $690.85   $690.80