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 $27.65   12.550%   $31.1201   $31.12   $31.10   $31.10  
  Purchase #2 $185.51   9.292%   $202.7476   $202.75   $202.75   $202.70  
  Purchase #3 $101.92   8.788%   $110.8767   $110.88   $110.90   $110.90  
  Purchase #4 $18.53   13.323%   $20.9988   $21.00   $21.00   $21.00  
  Purchase #5 $240.45   8.165%   $260.0827   $260.08   $260.10   $260.10  
  Purchase #6 $138.65   3.577%   $143.6095   $143.61   $143.60   $143.60  
  Purchase #7 $96.57   12.870%   $108.9986   $109.00   $109.00   $109.00  
  7 purchase totals = $809.28       $878.4340   $878.44   $878.45   $878.40