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 $28.50   11.652%   $31.8208   $31.82   $31.80   $31.80  
  Purchase #2 $137.89   13.653%   $156.7161   $156.72   $156.70   $156.70  
  Purchase #3 $49.46   8.668%   $53.7472   $53.75   $53.75   $53.70  
  Purchase #4 $100.41   7.756%   $108.1978   $108.20   $108.20   $108.20  
  Purchase #5 $105.99   5.206%   $111.5078   $111.51   $111.50   $111.50  
  Purchase #6 $36.98   3.247%   $38.1807   $38.18   $38.20   $38.20  
  Purchase #7 $127.44   8.261%   $137.9678   $137.97   $137.95   $138.00  
  7 purchase totals = $586.67       $638.1382   $638.15   $638.10   $638.10