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 $234.71   13.437%   $266.248   $266.25   $266.25   $266.20  
  Purchase #2 $6.90   12.743%   $7.7793   $7.78   $7.80   $7.80  
  Purchase #3 $70.85   13.684%   $80.5451   $80.55   $80.55   $80.50  
  Purchase #4 $93.74   10.172%   $103.2752   $103.28   $103.30   $103.30  
  Purchase #5 $117.63   8.500%   $127.6286   $127.63   $127.65   $127.60  
  Purchase #6 $120.80   11.826%   $135.0858   $135.09   $135.10   $135.10  
  Purchase #7 $50.48   3.695%   $52.3452   $52.35   $52.35   $52.30  
  7 purchase totals = $695.11       $772.9072   $772.93   $773.00   $772.80