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 $25.94   12.282%   $29.126   $29.13   $29.15   $29.10  
  Purchase #2 $230.74   8.814%   $251.0774   $251.08   $251.10   $251.10  
  Purchase #3 $53.87   7.459%   $57.8882   $57.89   $57.90   $57.90  
  Purchase #4 $209.83   13.598%   $238.3627   $238.36   $238.35   $238.40  
  Purchase #5 $52.34   6.625%   $55.8075   $55.81   $55.80   $55.80  
  Purchase #6 $122.82   10.236%   $135.3919   $135.39   $135.40   $135.40  
  Purchase #7 $139.97   9.590%   $153.3931   $153.39   $153.40   $153.40  
  7 purchase totals = $835.51       $921.0468   $921.05   $921.10   $921.10