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 $81.99   6.175%   $87.0529   $87.05   $87.05   $87.10  
  Purchase #2 $122.70   12.809%   $138.4166   $138.42   $138.40   $138.40  
  Purchase #3 $39.98   4.152%   $41.64   $41.64   $41.65   $41.60  
  Purchase #4 $185.29   10.298%   $204.3712   $204.37   $204.35   $204.40  
  Purchase #5 $44.54   9.794%   $48.9022   $48.90   $48.90   $48.90  
  Purchase #6 $17.29   8.730%   $18.7994   $18.80   $18.80   $18.80  
  Purchase #7 $94.84   10.417%   $104.7195   $104.72   $104.70   $104.70  
  7 purchase totals = $586.63       $643.9018   $643.90   $643.85   $643.90