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 $239.93   6.212%   $254.8345   $254.83   $254.85   $254.80  
  Purchase #2 $24.29   12.464%   $27.3175   $27.32   $27.30   $27.30  
  Purchase #3 $67.55   10.447%   $74.6069   $74.61   $74.60   $74.60  
  Purchase #4 $138.29   11.699%   $154.4685   $154.47   $154.45   $154.50  
  Purchase #5 $136.52   12.404%   $153.4539   $153.45   $153.45   $153.50  
  Purchase #6 $197.31   7.585%   $212.276   $212.28   $212.30   $212.30  
  Purchase #7 $68.96   5.288%   $72.6066   $72.61   $72.60   $72.60  
  7 purchase totals = $872.85       $949.5639   $949.57   $949.55   $949.60