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 $87.70   6.638%   $93.5215   $93.52   $93.50   $93.50  
  Purchase #2 $120.78   13.134%   $136.6432   $136.64   $136.65   $136.60  
  Purchase #3 $10.69   12.816%   $12.06   $12.06   $12.05   $12.10  
  Purchase #4 $94.59   12.215%   $106.1442   $106.14   $106.15   $106.10  
  Purchase #5 $239.50   6.474%   $255.0052   $255.01   $255.00   $255.00  
  Purchase #6 $209.73   5.398%   $221.0512   $221.05   $221.05   $221.10  
  Purchase #7 $128.28   6.468%   $136.5772   $136.58   $136.60   $136.60  
  7 purchase totals = $891.27       $961.0025   $961.00   $961.00   $961.00