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 $59.41   8.305%   $64.344   $64.34   $64.35   $64.30  
  Purchase #2 $50.76   8.612%   $55.1315   $55.13   $55.15   $55.10  
  Purchase #3 $120.79   3.722%   $125.2858   $125.29   $125.30   $125.30  
  Purchase #4 $172.73   7.494%   $185.6744   $185.67   $185.65   $185.70  
  Purchase #5 $166.28   13.580%   $188.8608   $188.86   $188.85   $188.90  
  Purchase #6 $228.59   12.151%   $256.366   $256.37   $256.35   $256.40  
  Purchase #7 $112.43   9.419%   $123.0198   $123.02   $123.00   $123.00  
  7 purchase totals = $910.99       $998.6823   $998.68   $998.65   $998.70