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 $170.31   4.742%   $178.3861   $178.39   $178.40   $178.40  
  Purchase #2 $130.68   11.366%   $145.5331   $145.53   $145.55   $145.50  
  Purchase #3 $19.75   5.364%   $20.8094   $20.81   $20.80   $20.80  
  Purchase #4 $128.31   7.839%   $138.3682   $138.37   $138.35   $138.40  
  Purchase #5 $145.42   6.569%   $154.9726   $154.97   $154.95   $155.00  
  Purchase #6 $102.36   7.483%   $110.0196   $110.02   $110.00   $110.00  
  Purchase #7 $107.72   5.326%   $113.4572   $113.46   $113.45   $113.50  
  7 purchase totals = $804.55       $861.5462   $861.55   $861.50   $861.60