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 $123.53   6.817%   $131.951   $131.95   $131.95   $132.00  
  Purchase #2 $51.78   13.294%   $58.6636   $58.66   $58.65   $58.70  
  Purchase #3 $239.34   9.572%   $262.2496   $262.25   $262.25   $262.20  
  Purchase #4 $143.55   11.771%   $160.4473   $160.45   $160.45   $160.40  
  Purchase #5 $120.52   11.866%   $134.8209   $134.82   $134.80   $134.80  
  Purchase #6 $100.29   4.499%   $104.802   $104.80   $104.80   $104.80  
  Purchase #7 $38.69   8.312%   $41.9059   $41.91   $41.90   $41.90  
  7 purchase totals = $817.70       $894.8403   $894.84   $894.80   $894.80