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 $121.63   7.392%   $130.6209   $130.62   $130.60   $130.60  
  Purchase #2 $36.28   8.821%   $39.4803   $39.48   $39.50   $39.50  
  Purchase #3 $126.28   5.220%   $132.8718   $132.87   $132.85   $132.90  
  Purchase #4 $40.51   12.648%   $45.6337   $45.63   $45.65   $45.60  
  Purchase #5 $77.79   6.246%   $82.6488   $82.65   $82.65   $82.60  
  Purchase #6 $87.44   11.390%   $97.3994   $97.40   $97.40   $97.40  
  Purchase #7 $13.45   9.474%   $14.7243   $14.72   $14.70   $14.70  
  7 purchase totals = $503.38       $543.3792   $543.37   $543.35   $543.30