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 $134.56   9.518%   $147.3674   $147.37   $147.35   $147.40  
  Purchase #2 $28.30   12.349%   $31.7948   $31.79   $31.80   $31.80  
  Purchase #3 $135.30   11.700%   $151.1301   $151.13   $151.15   $151.10  
  Purchase #4 $8.38   11.216%   $9.3199   $9.32   $9.30   $9.30  
  Purchase #5 $98.98   10.234%   $109.1096   $109.11   $109.10   $109.10  
  Purchase #6 $6.75   9.331%   $7.3798   $7.38   $7.40   $7.40  
  Purchase #7 $174.34   3.744%   $180.8673   $180.87   $180.85   $180.90  
  7 purchase totals = $586.61       $636.9689   $636.97   $636.95   $637.00