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 $11.29   6.213%   $11.9914   $11.99   $12.00   $12.00  
  Purchase #2 $73.96   13.276%   $83.7789   $83.78   $83.80   $83.80  
  Purchase #3 $181.58   7.283%   $194.8045   $194.80   $194.80   $194.80  
  Purchase #4 $69.95   7.672%   $75.3166   $75.32   $75.30   $75.30  
  Purchase #5 $80.99   11.679%   $90.4488   $90.45   $90.45   $90.40  
  Purchase #6 $145.95   11.553%   $162.8116   $162.81   $162.80   $162.80  
  Purchase #7 $17.71   5.746%   $18.7276   $18.73   $18.75   $18.70  
  7 purchase totals = $581.43       $637.8794   $637.88   $637.90   $637.80