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 $83.45   8.311%   $90.3855   $90.39   $90.40   $90.40  
  Purchase #2 $75.93   6.358%   $80.7576   $80.76   $80.75   $80.80  
  Purchase #3 $151.83   6.721%   $162.0345   $162.03   $162.05   $162.00  
  Purchase #4 $73.75   8.470%   $79.9966   $80.00   $80.00   $80.00  
  Purchase #5 $8.47   9.774%   $9.2979   $9.30   $9.30   $9.30  
  Purchase #6 $224.86   3.399%   $232.503   $232.50   $232.50   $232.50  
  Purchase #7 $150.31   13.401%   $170.453   $170.45   $170.45   $170.50  
  7 purchase totals = $768.60       $825.4281   $825.43   $825.45   $825.50