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 $85.26   10.158%   $93.9207   $93.92   $93.90   $93.90  
  Purchase #2 $77.84   11.424%   $86.7324   $86.73   $86.75   $86.70  
  Purchase #3 $54.79   9.497%   $59.9934   $59.99   $60.00   $60.00  
  Purchase #4 $10.45   3.345%   $10.7996   $10.80   $10.80   $10.80  
  Purchase #5 $109.28   7.616%   $117.6028   $117.60   $117.60   $117.60  
  Purchase #6 $59.96   4.259%   $62.5137   $62.51   $62.50   $62.50  
  Purchase #7 $69.64   3.135%   $71.8232   $71.82   $71.80   $71.80  
  7 purchase totals = $467.22       $503.3858   $503.37   $503.35   $503.30