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 $238.39   6.128%   $252.9985   $253.00   $253.00   $253.00  
  Purchase #2 $59.72   4.370%   $62.3298   $62.33   $62.35   $62.30  
  Purchase #3 $27.68   11.287%   $30.8042   $30.80   $30.80   $30.80  
  Purchase #4 $113.98   9.468%   $124.7716   $124.77   $124.75   $124.80  
  Purchase #5 $206.68   9.336%   $225.9756   $225.98   $226.00   $226.00  
  Purchase #6 $86.89   9.124%   $94.8178   $94.82   $94.80   $94.80  
  Purchase #7 $31.85   8.797%   $34.6518   $34.65   $34.65   $34.70  
  7 purchase totals = $765.19       $826.3493   $826.35   $826.35   $826.40