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 $19.73   5.792%   $20.8728   $20.87   $20.85   $20.90  
  Purchase #2 $136.60   9.694%   $149.842   $149.84   $149.85   $149.80  
  Purchase #3 $233.91   4.122%   $243.5518   $243.55   $243.55   $243.60  
  Purchase #4 $28.96   3.417%   $29.9496   $29.95   $29.95   $29.90  
  Purchase #5 $47.52   9.392%   $51.9831   $51.98   $52.00   $52.00  
  Purchase #6 $20.72   6.283%   $22.0218   $22.02   $22.00   $22.00  
  Purchase #7 $120.91   13.734%   $137.5158   $137.52   $137.50   $137.50  
  7 purchase totals = $608.35       $655.7369   $655.73   $655.70   $655.70