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 $136.91   12.453%   $153.9594   $153.96   $153.95   $154.00  
  Purchase #2 $113.56   11.365%   $126.4661   $126.47   $126.45   $126.50  
  Purchase #3 $11.70   13.268%   $13.2524   $13.25   $13.25   $13.30  
  Purchase #4 $181.39   5.550%   $191.4571   $191.46   $191.45   $191.50  
  Purchase #5 $60.54   3.822%   $62.8538   $62.85   $62.85   $62.90  
  Purchase #6 $208.62   12.331%   $234.3449   $234.34   $234.35   $234.30  
  Purchase #7 $178.38   9.412%   $195.1691   $195.17   $195.15   $195.20  
  7 purchase totals = $891.10       $977.5028   $977.50   $977.45   $977.70