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 $27.98   8.246%   $30.2872   $30.29   $30.30   $30.30  
  Purchase #2 $85.30   5.521%   $90.0094   $90.01   $90.00   $90.00  
  Purchase #3 $107.94   3.850%   $112.0957   $112.10   $112.10   $112.10  
  Purchase #4 $110.67   5.702%   $116.9804   $116.98   $117.00   $117.00  
  Purchase #5 $197.88   3.494%   $204.7939   $204.79   $204.80   $204.80  
  Purchase #6 $15.70   7.606%   $16.8941   $16.89   $16.90   $16.90  
  Purchase #7 $180.43   9.228%   $197.0801   $197.08   $197.10   $197.10  
  7 purchase totals = $725.90       $768.1408   $768.14   $768.20   $768.20