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 $223.77   13.837%   $254.7331   $254.73   $254.75   $254.70  
  Purchase #2 $13.42   6.190%   $14.2507   $14.25   $14.25   $14.30  
  Purchase #3 $97.55   11.286%   $108.5595   $108.56   $108.55   $108.60  
  Purchase #4 $36.64   12.410%   $41.187   $41.19   $41.20   $41.20  
  Purchase #5 $199.55   12.742%   $224.9767   $224.98   $225.00   $225.00  
  Purchase #6 $174.70   5.391%   $184.1181   $184.12   $184.10   $184.10  
  Purchase #7 $232.97   12.186%   $261.3597   $261.36   $261.35   $261.40  
  7 purchase totals = $978.60       $1,089.1848   $1,089.19   $1,089.20   $1,089.30