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 $204.57   12.471%   $230.0819   $230.08   $230.10   $230.10  
  Purchase #2 $50.63   3.421%   $52.3621   $52.36   $52.35   $52.40  
  Purchase #3 $35.95   4.421%   $37.5393   $37.54   $37.55   $37.50  
  Purchase #4 $110.50   11.240%   $122.9202   $122.92   $122.90   $122.90  
  Purchase #5 $101.69   11.597%   $113.483   $113.48   $113.50   $113.50  
  Purchase #6 $182.84   3.748%   $189.6928   $189.69   $189.70   $189.70  
  Purchase #7 $56.83   5.585%   $60.004   $60.00   $60.00   $60.00  
  7 purchase totals = $743.01       $806.0833   $806.07   $806.10   $806.10