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 $55.38   10.119%   $60.9839   $60.98   $61.00   $61.00  
  Purchase #2 $226.84   5.309%   $238.8829   $238.88   $238.90   $238.90  
  Purchase #3 $164.36   8.568%   $178.4424   $178.44   $178.45   $178.40  
  Purchase #4 $181.93   12.540%   $204.744   $204.74   $204.75   $204.70  
  Purchase #5 $22.54   7.328%   $24.1917   $24.19   $24.20   $24.20  
  Purchase #6 $126.99   9.641%   $139.2331   $139.23   $139.25   $139.20  
  Purchase #7 $145.67   6.711%   $155.4459   $155.45   $155.45   $155.40  
  7 purchase totals = $923.71       $1,001.9239   $1,001.91   $1,002.00   $1,001.80