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 $27.90   7.737%   $30.0586   $30.06   $30.05   $30.10  
  Purchase #2 $18.95   13.123%   $21.4368   $21.44   $21.45   $21.40  
  Purchase #3 $234.75   7.295%   $251.875   $251.88   $251.90   $251.90  
  Purchase #4 $127.60   9.124%   $139.2422   $139.24   $139.25   $139.20  
  Purchase #5 $129.45   7.516%   $139.1795   $139.18   $139.20   $139.20  
  Purchase #6 $123.52   7.507%   $132.7926   $132.79   $132.80   $132.80  
  Purchase #7 $95.73   3.502%   $99.0825   $99.08   $99.10   $99.10  
  7 purchase totals = $757.90       $813.6672   $813.67   $813.75   $813.70