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 $34.70   12.116%   $38.9043   $38.90   $38.90   $38.90  
  Purchase #2 $231.76   4.332%   $241.7998   $241.80   $241.80   $241.80  
  Purchase #3 $203.26   13.380%   $230.4562   $230.46   $230.45   $230.50  
  Purchase #4 $221.89   11.290%   $246.9414   $246.94   $246.95   $246.90  
  Purchase #5 $157.89   5.222%   $166.135   $166.14   $166.15   $166.10  
  Purchase #6 $55.34   13.521%   $62.8225   $62.82   $62.80   $62.80  
  Purchase #7 $244.74   6.267%   $260.0779   $260.08   $260.10   $260.10  
  7 purchase totals = $1,149.58       $1,247.1371   $1,247.14   $1,247.15   $1,247.10