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 $7.65   4.541%   $7.9974   $8.00   $8.00   $8.00  
  Purchase #2 $9.73   3.587%   $10.079   $10.08   $10.10   $10.10  
  Purchase #3 $128.51   6.464%   $136.8169   $136.82   $136.80   $136.80  
  Purchase #4 $86.90   9.166%   $94.8653   $94.87   $94.85   $94.90  
  Purchase #5 $106.53   9.197%   $116.3276   $116.33   $116.35   $116.30  
  Purchase #6 $87.36   4.842%   $91.59   $91.59   $91.60   $91.60  
  Purchase #7 $107.54   4.466%   $112.3427   $112.34   $112.35   $112.30  
  7 purchase totals = $534.22       $570.0189   $570.03   $570.05   $570.00