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 $164.91   7.678%   $177.5718   $177.57   $177.55   $177.60  
  Purchase #2 $121.68   6.625%   $129.7413   $129.74   $129.75   $129.70  
  Purchase #3 $5.90   10.431%   $6.5154   $6.52   $6.50   $6.50  
  Purchase #4 $31.77   13.679%   $36.1158   $36.12   $36.10   $36.10  
  Purchase #5 $25.78   3.503%   $26.6831   $26.68   $26.70   $26.70  
  Purchase #6 $29.91   4.439%   $31.2377   $31.24   $31.25   $31.20  
  Purchase #7 $215.67   7.153%   $231.0969   $231.10   $231.10   $231.10  
  7 purchase totals = $595.62       $638.9620   $638.97   $638.95   $638.90