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 $59.93   10.201%   $66.0435   $66.04   $66.05   $66.00  
  Purchase #2 $111.66   7.451%   $119.9798   $119.98   $120.00   $120.00  
  Purchase #3 $240.43   9.504%   $263.2805   $263.28   $263.30   $263.30  
  Purchase #4 $112.83   6.868%   $120.5792   $120.58   $120.60   $120.60  
  Purchase #5 $43.31   13.502%   $49.1577   $49.16   $49.15   $49.20  
  Purchase #6 $247.52   7.391%   $265.8142   $265.81   $265.80   $265.80  
  Purchase #7 $132.61   7.165%   $142.1115   $142.11   $142.10   $142.10  
  7 purchase totals = $948.29       $1,026.9664   $1,026.96   $1,027.00   $1,027.00