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 $223.63   12.651%   $251.9214   $251.92   $251.90   $251.90  
  Purchase #2 $30.96   11.254%   $34.4442   $34.44   $34.45   $34.40  
  Purchase #3 $11.55   9.574%   $12.6558   $12.66   $12.65   $12.70  
  Purchase #4 $182.89   6.349%   $194.5017   $194.50   $194.50   $194.50  
  Purchase #5 $208.29   5.421%   $219.5814   $219.58   $219.60   $219.60  
  Purchase #6 $231.61   6.794%   $247.3456   $247.35   $247.35   $247.30  
  Purchase #7 $9.81   9.769%   $10.7683   $10.77   $10.75   $10.80  
  7 purchase totals = $898.74       $971.2184   $971.22   $971.20   $971.20