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 $48.78   13.386%   $55.3097   $55.31   $55.30   $55.30  
  Purchase #2 $168.41   4.636%   $176.2175   $176.22   $176.20   $176.20  
  Purchase #3 $116.43   3.775%   $120.8252   $120.83   $120.85   $120.80  
  Purchase #4 $159.76   4.564%   $167.0514   $167.05   $167.05   $167.10  
  Purchase #5 $83.36   5.776%   $88.1749   $88.17   $88.15   $88.20  
  Purchase #6 $212.62   5.390%   $224.0802   $224.08   $224.10   $224.10  
  Purchase #7 $125.93   10.149%   $138.7106   $138.71   $138.70   $138.70  
  7 purchase totals = $915.29       $970.3695   $970.37   $970.35   $970.40